When you deploy Exchange 2007 Unified Messaging, your users will have access to their e-mail, voice mail, and fax messages from either Microsoft Office Outlook 2007 or the version of Outlook Web Access that is included with Exchange 2007. Additionally, users will be able to use the following features:

• Access to Exchange information   To offer a seamless voice mail experience for the user, UM-enabled users can access a full set of voice mail features from Windows Mobile powered devices, Outlook 2007, and Outlook Web Access. These features include many voice mail configuration options and the ability to play a voice message from either the reading pane by using an integrated Windows Media Player or from the message list by using their computer speakers.
  
• Play on Phone   The Play on Phone feature lets UM-enabled users play voice messages over a telephone. If a UM-enabled user sits in an office cubicle, is using a public computer or a computer that is not enabled for multimedia, or is listening to a voice message that is confidential, the user might not want to or be able to listen to a voice message through their computer speakers. Alternatively, they can play the voice message by using any telephone, including home, office, or cellular telephones.
  
• Voice mail form   The Outlook 2007 voice mail form resembles the default e-mail form. It gives users an interface for performing actions such as playing, stopping, or pausing voice messages, playing voice messages on a telephone, and adding and editing notes.
  The voice mail form includes the embedded Windows Media Player and an audio notes field. The embedded Player and notes field are displayed in either the preview pane when a user previews a voice message or in a separate window when they open the voice message. If a user is not enabled for Unified Messaging or Outlook 2007 has not been installed on the client computer, they view voice messages as e-mail attachments, and the voice mail form is not available.
  
• Fax receiving   Exchange 2007 Unified Messaging enables voice messages to be delivered into a user's Exchange 2007 mailbox and also lets users receive fax messages in their mailbox. A fax message is sent to the user's mailbox as an e-mail message that has an image file that has a .tif extension attached. When a user receives such a message in their mailbox, they can open the attached file by using a software application that can open and view image files that have a .tif extension.
  
• User configuration   A user who is enabled for Unified Messaging can configure several voice mail options for Unified Messaging by using Outlook Web Access. For example, the user can configure telephone access numbers and the voice mail Play on Phone number, and can reset a voice mail access PIN.
  
• Call answering   Call answering includes answering an incoming call on behalf of a user, playing their personal greeting, recording a message, and submitting it for delivery to their Inbox as an e-mail message.
  
• Outlook Voice Access   There are two Unified Messaging user interfaces available to UM-enabled users or subscribers: the Telephone User Interface (TUI) and the Voice User Interface (VUI). In Exchange 2007, these two interfaces together are called Outlook Voice Access. A subscriber can use Outlook Voice Access when they access the Unified Messaging system from an external or internal telephone. They can use Outlook Voice Access to access their Exchange 2007 mailbox, including their personal e-mail, voice messages, and calendar information. Users can listen to, reply to, create, and forward unread e-mail messages by using the telephone.
  
• Subscriber access   The subscriber access feature enables dial-in access for your organization's users. UM-enabled users or subscribers who dial in to the Unified Messaging system can access their mailbox by using Outlook Voice Access. By using a telephone, a subscriber or user can:
  
  • Access voice mail.
    
  • Listen, forward, or reply to e-mail messages.
    
  • Listen to calendar information.
    
  • Access or dial contacts who are stored in the global address list or a personal contact list.
    
  • Accept or cancel meeting requests.
    
  • Set a voice mail Out-of-Office message.
    
  • Set user security preferences and personal options.
    
• Auto attendant   An auto attendant is a set of voice prompts that gives external or internal users access to the Exchange 2007 Unified Messaging system. Users can use the telephone keypad or speech inputs to move through the auto attendant menu structure, place a call to a user, or locate a user and then place a call to that user. An auto attendant gives the administrator the ability to:
  
  • Create a customized menu for external users.
    
  • Define informational greetings, business hours greetings, and non-business hours greetings.
    
  • Define holiday schedules.
    
  • Describe how to search the organization's directory.
    
  • Describe how to connect to a user's extension so external callers can call a user by specifying their extension.
    
  • Describe how to search the organization's directory so external callers can search the organization's directory and call a specific user.
    
  • Enable external users to call the operator.
    

Deploying Exchange 2007 Unified Messaging offers the following benefits to administrators:

• A complete unified messaging system   Exchange 2007 Unified Messaging offers a true unified messaging solution by using a single store, transport, and directory infrastructure. The store is provided by the Exchange 2007 Mailbox server role. The transport is provided by the Exchange 2007 Hub Transport server role. By locating all e-mail and voice mail messages in a single store, you can manage this information from a single management point. For example, all e-mail, voice mail, and fax messages can be backed up by using a single administration interface and toolset. This greatly reduces the overall cost of administration by consolidating infrastructure and training.
  
• An Exchange 2007 deployment and administration model   By using the Exchange 2007 Unified Messaging solution, you take advantage of the Exchange 2007 server design. You are not required to purchase a new server to run the Unified Messaging server role. More important, you can reuse your Exchange knowledge, including training and troubleshooting methodology, and apply it to managing your voice mail and fax messaging infrastructure. 
  
• An Exchange 2007 security model   The Microsoft Exchange Unified Messaging service runs as an Exchange server account. This means that you do not have to create or manage a super user account for Exchange 2007 Unified Messaging.
  
• Consolidation of voice mail systems   Currently, most voice messaging systems require that all the voice messaging system components be installed in every physical office location in an organization. In this kind of arrangement, the voice messaging systems in those branch offices are located outside the central office and must be administered onsite. This distributed management scenario can frequently result in increased administration costs and complexity. Exchange 2007 Unified Messaging lets you manage your voice mail system from a central location. To create a centralized management system for Unified Messaging, you can place all Unified Messaging servers in a datacenter or location, and then deploy IP gateways in each of your branch offices that replace the voice messaging system for each branch office. Deploying a centralized voice messaging system in this manner can result in a significant savings in hardware and administrative costs.
  
• Speech enabled auto attendants   When internal or external callers call in to the Exchange 2007 Unified Messaging system, a series of voice prompts assists them in moving through the menu system called an auto attendant. The auto attendant enables the caller to connect to a person in an organization or locate a person in the organization so that they can place a call without assistance from a human operator. With an auto attendant, callers hear voice prompts instead of a human operator or receptionist. 
  You can create multiple auto attendants in Exchange 2007 Unified Messaging. Within the Active Directory directory service, each auto attendant is represented as an object. Configuration settings for an auto attendant are made on the Active Directory object and can include language settings, customized menus, and other menu navigation settings. You can also configure each UM auto attendant so that when a user calls in to an auto attendant, the caller can use either dual tone multi-frequency (DTMF) inputs (also known as touchtone inputs) or voice inputs (available in English) to move through the Unified Messaging auto attendant menu system.
  

Unified Messaging dial plans are integral to the operation of Exchange 2007 Unified Messaging and are required to successfully deploy Unified Messaging on your network. A Unified Messaging dial plan is an Active Directory container object that logically represents one or more PBXs that share common user extension numbers. In practical terms, users' extensions that are attached to PBXs share a common numbering plan. Users can dial one another's telephone extensions without dialing a full telephone number. A UM dial plan is a logical representation of a telephony dial plan.

UM dial plans in Exchange 2007 Unified Messaging require that user telephone extension numbers be unique. UM dial plans are implemented in Unified Messaging to ensure that user telephone extensions are unique. In some telephony networks, multiple PBXs and multiple dial plans can exist. In these telephony networks, there could be two different users who have identical telephone extensions. UM dial plans resolve this issue. You can put the two users who have the same extension number into two separate UM dial plans. This makes the users' numbers unique for Unified Messaging.

The following figure illustrates how Unified Messaging dial plans can be used in an organization that has a single forest and multiple physical sites.

How Dial Plans Work

When you integrate a telephony network together with Unified Messaging, there must be an IP gateway that connects your telephony network with your IP-based network. Each IP gateway in your organization is represented by a UM IP gateway object in Active Directory. Exchange 2007 Unified Messaging requires that you create at least one UM dial plan and that the UM dial plan has a Unified Messaging server and a UM IP gateway associated with it. After you install the Unified Messaging server role on a computer that is running Exchange 2007, you must associate the Unified Messaging server with at least one UM dial plan before it will answer calls. You can also associate a single Unified Messaging server with multiple UM dial plans. After the Unified Messaging server is associated with a UM dial plan, you must create a UM IP gateway and associate it with the UM dial plan that was created.

If you create the first UM IP gateway and do not specify a UM dial plan at the time that you create it, a default UM hunt group is also created. Creating and associating these objects in Active Directory enables the Unified Messaging server to receive calls from the IP gateway and then process incoming calls for users who are associated with the UM dial plan. When a call comes in to the IP gateway, the IP gateway forwards the call to a Unified Messaging server and the Unified Messaging server tries to match the extension number of the user to the associated UM dial plan. The dial plan is identified from the pilot numbers to which the gateway presents the call.

A UM IP Gateway object is a container object that logically represents a physical IP gateway hardware device, IP-PBX, or other SIP server that can interoperate with Exchange Unified Messaging. Before the IP gateway can be used to process Unified Messaging calls, it must be represented by an object in Active Directory.

Although there are many types and manufacturers of PBXs, IP gateways, and IP PBXs, there are basically two types of IP gateway component configurations:

• IP PBX   A single device
  
• PBX (legacy) and an IP gateway   Two separate components
  

To support Exchange 2007 Unified Messaging, one or both types of IP device configurations are used when connecting a telephony network infrastructure to a data network infrastructure.

UM IP Gateway Objects

The UM IP Gateway object is an Active Directory container object that contains one or more Active Directory UM hunt groups and other UM IP gateway configuration settings. UM IP Gateway objects are created in Active Directory to logically represent a physical hardware device known as an IP gateway. The UM IP gateway can represent either an IP gateway or an IP PBX. The combination of the IP Gateway object and a UM Hunt Group object establishes a logical link between an IP gateway hardware device and a UM dial plan.

After the UM IP Gateway object is created, the IP gateway can be linked to or associated with a single or multiple UM hunt groups and UM dial plans. The UM hunt group provides a link between the UM IP gateway and a UM dial plan. By creating multiple UM hunt groups, you can associate a single UM IP gateway with multiple UM dial plans.

Enabling and Disabling UM IP Gateways

By default, UM IP gateways are left in an enabled state after they are created. However, the UM IP gateway can be enabled or disabled. If you disable a UM IP gateway, it can be in one of two disabled modes. The first disabled mode forces all associated Unified Messaging servers to drop existing calls. The second disabled mode forces the Unified Messaging server that is associated with the UM IP gateway to stop handling any new calls presented by the IP gateway.

A hunt group describes a set of PBX or IP PBX resources or extension numbers that are shared by users. Hunt groups are used to efficiently distribute calls into or out of a given business unit. For example, a PBX or IP PBX might be configured to have 10 extension numbers for the sales department. The 10 sales extension numbers would be configured as one hunt group. In a PBX or IP PBX, hunt groups are used to efficiently locate an open line, extension, or channel when an incoming call is received.

In a telephony network, a hunt group is defined as a set of extension numbers that are grouped as a single logical unit. When an incoming call is received, the PBX or IP PBX uses the hunt group or the group of extensions that are defined to "hunt" for an available or open line, extension, or channel that can be used to receive the call.

There are multiple algorithms or methods that have been created to be used by a PBX or IP PBX to define how the open line, extension, or channel will be located. These include the following:

• Round robin
  
• Most idle
  
• Start with lowest number
  

Creating and defining a hunt group in a PBX or IP PBX reduces the possibility that a caller who places an incoming call will receive a busy signal when the call is received.

Understanding UM Hunt Groups

Unified Messaging hunt groups are very important to the operation of the Unified Messaging system. The UM hunt group is a logical representation of an existing PBX or IP PBX hunt group. UM hunt groups act as a connection or link between the UM IP gateway and the UM dial plan. A single UM hunt group is associated with at least one UM IP gateway and one UM dial plan.

Unified Messaging hunt groups are used to define the PBX or IP PBX hunt group from which incoming calls will be received. A pilot number that is defined for a hunt group in the PBX or IP PBX must also be defined within the UM hunt group. The pilot number is used to match the information presented for incoming calls through the Session Initiation Protocol (SIP) header on the message. The pilot number enables the Unified Messaging server to interpret the call together with the correct dial plan so that the voice or fax message can be routed correctly. It is very important to configure Unified Messaging hunt groups correctly, because incoming calls that do not correctly match the pilot number defined on the UM hunt group will not be answered and incoming call routing will fail.

When you create a Unified Messaging hunt group, you are enabling all Unified Messaging servers that are specified within the UM dial plan to communicate with an IP gateway. If you delete the Unified Messaging hunt group, the associated IP gateway will no longer service calls by using the specified pilot number. If the IP gateway is left without remaining UM hunt groups, the IP gateway will be unable to handle incoming calls.

UM mailbox policies are required when you enable users for Exchange 2007 Unified Messaging. They are useful for applying and standardizing Unified Messaging configuration settings for UM-enabled users. You create UM mailbox policies to apply a common set of policies or security settings to a collection of UM-enabled mailboxes. You use Unified Messaging mailbox policies to set Unified Messaging settings for UM-enabled users, such as the following:

• PIN policies
  
• Dialing restrictions
  
• Other general UM mailbox policy properties
  

For example, you can create a UM mailbox policy to increase the level of PIN security by reducing the maximum number of logon failures before a user will be locked out for a specific group of UM-enabled users, such as executives.

Unified Messaging mailbox policies are created in the Configuration container in Active Directory by using the Exchange Management Shell or the Exchange Management Console. By default, a single UM mailbox policy is created every time that you create a UM dial plan. The new UM mailbox policy is associated with the UM dial plan and part of the dial plan name is included in the display name of the UM mailbox policy. However, you can create additional UM mailbox policies based on the needs of your organization. Although a single UM mailbox policy is required to enable users for Unified Messaging, you can create additional UM mailbox policies and apply a common set of mailbox policy settings for other groups of users.

The mailbox of each UM-enabled user must be linked to a single UM mailbox policy when the mailbox is enabled for Unified Messaging. After you create a UM mailbox policy, you link one or more UM-enabled mailboxes to the UM mailbox policy. This lets you control PIN security settings such as the minimum number of digits in a PIN or the maximum number of logon attempts for the UM-enabled users who are associated with the UM mailbox policy. If you prefer, you can also control message text settings or dialing restrictions for the same or a different group of UM-enabled mailboxes.

Multiple UM-enabled users can be linked to a single UM mailbox policy. However, a single user can be associated with only one UM mailbox policy. After you create a new UM mailbox policy and link it to a UM dial plan, the UM mailbox policy settings that are defined are applied to the UM-enabled users. The settings that are defined on a UM dial plan and a UM mailbox policy will be applied to all users who are associated with the UM mailbox policy.

The following figure illustrates how Unified Messaging mailbox policies can be created to control dialing restrictions and PIN security settings for three different groups.

Exchange 2007 Unified Messaging enables you to create one or more UM auto attendants, depending on the needs of your organization. UM auto attendants can be used to create a voice menu system for an organization that lets external and internal callers move through the UM auto attendant menu system to locate and place or transfer calls to company users or departments in an organization.

When anonymous or unauthenticated users call an external business telephone number, or when internal callers call a defined extension number, they are presented with a series of voice prompts that help them place a call to a user or locate a user in the organization and then place a call to that user. The UM auto attendant is a series of voice prompts or .wav files that callers hear instead of a human operator when they call an organization that has Exchange 2007 Unified Messaging. The UM auto attendant lets callers move through the menu system, place calls, or locate users by using DTMF or voice inputs. However, for automatic speech recognition (ASR) or voice inputs to be used, you must enable ASR on the UM auto attendant.

A UM auto attendant has the following features:

• It provides corporate or informational greetings.
  
• It provides custom corporate menus. You can link a menu option to another auto attendant to have more than one level.
  
• It provides a directory search function that enables a caller to search an organization's directory for a name.
  
• It enables a caller to connect to the telephone of, or leave a message for, members of the organization.
  

In Active Directory, each UM auto attendant that is created is represented as an object. There is no limit to the number of UM auto attendants that you can create in Active Directory. Each auto attendant can support an unlimited number of extensions. A UM auto attendant is associated with one, and only one, UM dial plan. However, UM auto attendants can reference or link to other UM auto attendants.

An incoming call that is received from an external telephone number or an internal telephone extension is processed by a Unified Messaging server and then sent to a UM auto attendant that has been created. The UM auto attendant is configured by the system administrator to use prerecorded voice (.wav) files that are then played over the telephone to the caller and that enable the caller to move through the Unified Messaging menu system. When you configure a UM auto attendant, you can customize all the .wav files that are used to meet the needs of your organization. For more information about custom prompts in Unified Messaging, see Understanding Unified Messaging Audio Prompts.

For more information about message flow with UM auto attendants, see Unified Messaging Auto Attendant Call Processing.

Auto Attendant with Multiple Languages

There are situations in which you may have to provide callers with auto attendants that have different languages. The language setting that is available on a UM auto attendant enables you to configure the default prompt language on the auto attendant. When you are using the default system prompts for the auto attendant, the default prompt language is the language that the caller will hear when the auto attendant answers the incoming call. This language setting will affect only the default system prompts that are provided when the Unified Messaging server role is installed. This setting will not affect custom prompts that have been configured on an auto attendant. The language that is selected as the default for the auto attendant is based on the version of Exchange 2007 that is installed.

When you install the U.S.-English version of Exchange 2007, there will be only one language available to configure on UM auto attendants: U.S. English. However, if you install a localized version of Exchange 2007, for example, Japanese, you will be able to configure the auto attendant that you create to use Japanese or U.S. English as the default language. Additional UM language packs can be installed on a Unified Messaging server to enable you to use other default language options on an auto attendant.

For example, if you have a business that is based in the United States but requires a menu system that gives callers the options of moving through the system in U.S. English, Spanish, and French, you must first install the UM language packs that you need. In this case, if you have installed the U.S.-English version of Exchange 2007, you would install the UM language packs for Spanish and French. However, because a Unified Messaging auto attendant can have only one language configured at a time, you would create four auto attendants: a main auto attendant that is configured to use U.S. English and then one auto attendant for each language: U.S. English, Spanish, and French. You would then configure the main auto attendant to have the appropriate key mappings to access the other auto attendants that you have created for each language. In this example, the main auto attendant would answer the incoming call and the caller would hear, "Welcome to Contoso, Ltd. For English, press or say 1. For Spanish, press or say 2. For French, press or say 3."

Auto Attendant Examples

The following examples demonstrate how you can use UM auto attendants together with Exchange 2007 Unified Messaging:

• Example 1   At a company called Contoso, Ltd., external customers can use three external telephone numbers: 425-555-1111 (Corporate Offices), 425-555-2222 (Product Support), and 425-555-3333 (Sales). The Human Resources, Administration, and Accounting departments have internal telephone extensions and must be accessed from the Corporate Offices UM auto attendant.
  To create a UM auto attendant structure that supports this scenario, create and configure three UM auto attendants that have the appropriate external telephone numbers. Create three other UM auto attendants for each department in the Corporate Offices. Then you configure each UM auto attendant based on your requirements, such as the greeting type or other navigational information.
  The following figure is a graphical representation of how Unified Messaging auto attendants can be used in Example 1.
  
  How to configure multiple UM auto attendants with multiple outside business telephone lines
  
• Example 2   At a company called Contoso, Ltd., external customers call one main number for the business, 425-555-1000. When an external caller calls the main number, the UM auto attendant answers and prompts the caller by saying, "Welcome to Contoso, Ltd. Please press or say 'One' to be transferred to corporate administration. Please press or say 'Two' to be transferred to product support. Please press or say 'Three' to be transferred to corporate information. Please press or say 'Zero' to be transferred to the operator." To create a UM auto attendant structure that supports this scenario, you create a UM auto attendant that has customized extensions that route the call to the appropriate extension number.
  The following figure is a graphical representation of how Unified Messaging auto attendants can be used in Example 2.
  
  How to configure multiple UM auto attendants with a single outside business telephone line
  

Voice and fax calls that come in to an Exchange 2007 organization can be placed by callers who are inside or outside the organization. When a caller places a call to a UM-enabled user's telephone extension and the user is unavailable to answer the call, the PBX forwards or routes the incoming call to an IP gateway and then to the Unified Messaging server. In a Unified Messaging system that uses an IP PBX, the IP PBX forwards the incoming message to the Unified Messaging server. The IP gateway or the IP PBX translates or converts the incoming audio stream into RTP for incoming voice messages or the T.38 protocol for incoming fax messages. The stream of IP data is then passed to the Unified Messaging server. After the Unified Messaging server receives the call, the Unified Messaging server processes the message and determines how to route the message.

The following figure illustrates how incoming voice and fax messages flow in an Exchange 2007 organization.

In the previous example, a call arrives at the PBX and is then forwarded to the Unified Messaging pilot number. The pilot number is mapped to the hunt group that goes to the IP gateway. The IP gateway presents the call to the Unified Messaging server. The Unified Messaging server then receives SIP invitation header information such as who the call is for, who the caller is, and why the call was redirected. The Unified Messaging server then looks up the number that was called and identifies which user was called and which dial plan the user belongs to. From there, the Unified Messaging server looks in Active Directory to see whether the user is enabled for UM. If the user is enabled for UM, the caller receives a greeting for the user. Users' greetings are securely stored in their mailbox. This lets them manage their greetings. After the caller reaches the user's voice mail greeting, they can leave a voice message that will be sent to the user's mailbox. The Unified Messaging server then records the message, locates a Hub Transport server, and submits the voice message to the Mailbox server that contains the UM-enabled user's mailbox.

When external or anonymous callers place a call by using an external business telephone number, or an internal anonymous caller places a call to an internal extension number, they are presented with voice prompts to help them move through the Unified Messaging menu system. The UM auto attendant is a set of voice prompts or .wav files that are played to callers instead of a human operator or receptionist when they call in to an organization that has Exchange 2007 Unified Messaging. Exchange 2007 Unified Messaging enables you to create one or more auto attendants, depending on the needs of your organization.

Auto Attendant Message Flow

When a call is received by a Unified Messaging server, the Unified Messaging server performs a Lightweight Directory Access Protocol (LDAP) query to an Active Directory domain controller to determine how to handle the incoming call.

The following figure illustrates the message flow when Unified Messaging auto attendants are used in an Exchange 2007 organization.

After you have created and enabled the auto attendant for your organization and associated it with a UM dial plan, the auto attendant can start to process calls. When a call for an auto attendant is received, it is first processed by the IP gateway and mapped to the appropriate hunt group. The call is then sent to the Unified Messaging server. The Unified Messaging server looks up the auto attendant object in Active Directory and then provides the correct auto attendant to handle the call.

With Exchange 2007 Unified Messaging, UM-enabled users or subscribers can access their e-mail, contacts, and calendaring information by using a standard analog, digital, or cellular telephone. When a UM-enabled user uses Outlook Voice Access, they can perform the following tasks:

• Listen to new and saved e-mail and voice mail messages.
  
• Forward, reply to, save, and delete e-mail and voice mail messages.
  
• Interact with their calendar.
  
• Locate a person in the global address list (GAL) or their personal contacts.
  
• Send a voice message to a person.
  
• Change their PIN, spoken name, or greetings.
  

Outlook Voice Access Message Flow

Incoming calls that are received by a Unified Messaging server from an Outlook Voice Access user are only routed to a Mailbox server to enable the user to access their mailbox. However, if a message is submitted by using Outlook Voice Access, for example, a change in the schedule of a meeting, the message is first submitted to a Hub Transport server within the same Active Directory site as the Unified Messaging server before it is routed to the recipient's mailbox.

The following figure illustrates how incoming calls and messages placed by subscribers or UM-enabled users flow in an Exchange 2007 organization.

When a user calls in to Outlook Voice Access, the PBX receives the call for the Unified Messaging number and the IP gateway presents the call to the Unified Messaging server. The Unified Messaging server then looks up the user account information in Active Directory and gathers information about the user. This information includes dial plan information. If the user places the call from their own extension, they are prompted for their PIN. The Unified Messaging server contacts the Mailbox server and authenticates the user. From there, the Unified Messaging server and the Mailbox server communicate information back to the user, as requested.

Every incoming call that is received from an IP gateway will generate IP-based network traffic and will consume some amount of your available network bandwidth. Prior to deploying Unified Messaging, you should perform an analysis of the network traffic to determine current usage patterns and identify any potential issues. On most networks, bandwidth demand is not evenly distributed throughout business hours. Because all the IP-based calls are routed directly to your Unified Messaging servers from the IP gateways on your network and this IP-based network traffic consumes some available bandwidth, you should follow these recommendations and guidelines:

• Place your PBXs physically close to your IP gateways.
  
• Place your IP gateways and your Unified Messaging servers on the same well connected network or within the same physical site.
  
• Place your Unified Messaging servers on the same well connected network or within the same physical site as other computers that have Exchange 2007 server roles installed, including Mailbox, Hub Transport, and Client Access servers.
  
• Terminate your Wide Area Network (WAN) connections close to where your telephony equipment is located.
  
• In branch office scenarios or over WAN connections, use the G.723.1 codec instead of the G.711u or G.711A codec to minimize the network traffic that is passed between your IP gateways and your Unified Messaging servers.
  

Generally, Unified Messaging scalability is determined by the number of concurrent calls. By default, a single Unified Messaging server can accept a maximum of 100 concurrent voice calls and 100 concurrent incoming fax calls. These calls can be either incoming or outgoing and can be generated when a user leaves a voice mail message, when an Outlook Voice Access user accesses their Exchange 2007 mailbox, or by a user who uses the Play on Phone feature to listen to their voice messages. Although the number of concurrent calls is an important factor to consider when you build a scalable Unified Messaging infrastructure, you must also determine the best codec to use to encode the voice messages and the types and number of users who you have to support.

• Number of concurrent calls   Although, by default, a Unified Messaging server can accept 100 concurrent voice messages and 100 concurrent fax messages, a single Unified Messaging server can be configured to accept a maximum of 200 concurrent voice messages and 200 concurrent incoming fax messages. When you increase the number of concurrent connections on a single Unified Messaging server, more system resources are required than if you decrease the number of concurrent connections. It is especially important to decrease this setting on low-end, slower computers on which the Unified Messaging server is installed. Performance counters are available and the Get-UMActiveCalls cmdlet can also be used to monitor the current number of concurrent calls that are connected to a Unified Messaging server.
  If the number of concurrent calls required by your organization is larger than the number that is supported by a single Unified Messaging server, you can scale horizontally and increase the capacity of concurrent calls by installing the Unified Messaging server role and then adding the Unified Messaging server to a dial plan.
  
• Voice mail storage codec   The term "codec" is a combination of the words "coding" and "decoding" and relates to digital data. A codec is a computer program or software that transforms digital data into an audio file format or streaming audio format.
  In Exchange Unified Messaging, there are two general types of codecs: the codec that is used between IP gateways and the codec that is used to encode voice messages. The Windows Media Audio (WMA), Group System Mobile (GSM) 06.10, and G.711 Pulse Code Modulation (PCM) Linear audio codecs are used to create .wma and .wav audio files for voice messages. However, the file type that is used depends on the audio codec that is used to create the voice message audio file. In Exchange Unified Messaging, the .wma audio codec creates .wma audio files and the GSM 06.10 and G.711 PCM Linear audio codecs produce .wav audio files. However, depending on the codec that is used, an audio file in .wma or .wav format is sent together with the e-mail message to the intended voice mail recipient. The size of Unified Messaging voice messages depends on the size of the attachment that holds the voice data. Additionally, the size of the attachment depends on the following factors:
  
  • The duration of the voice mail recording
    
  • The audio codec that is used
    
  • The audio file storage format
    
  The following figure illustrates how the size of the audio file depends on the duration of the voice mail recording for the three audio codecs that you can use in Unified Messaging.
  

  Note:
  In the following figure, the average length of a call-answered voice message is approximately 30 seconds.

  Audio file size
  
  The sample bit rate (bit/sec) and compression properties for each audio codec that is used in Unified Messaging are as follows:
  
  • WMA – 16-bit – compressed file
    
  • G.711 – 16-bit – uncompressed file
    
  • GSM 06.10 – 8-bit – compressed file
    
  Using the WMA codec, we estimate that each Unified Messaging server can handle 60 to 75 concurrent IP-based calls. This estimate is also based on the assumption that 14 percent of all the IP-based calls arrive during the single busiest hour of a day. Based on these two assumptions, and assuming that your users access their voice messages frequently by using Outlook Voice Access, we estimate that each Unified Messaging server can support between 2,000 and 10,000 users.
  Generally, you should allow for the following number of concurrent calls per Unified Messaging server:
  
  • 60 if the default dial plan codec is WMA
    
  • 75 if the default dial plan codec is GSM
    
• Types of users   There are two types of users who access the Unified Messaging system: authenticated users and unauthenticated users. These two types of users consume Unified Messaging resources. When you build a scalable Unified Messaging environment, you must consider the effect that these users will have and the resources that each of these users will consume.
  
  • Authenticated   Authenticated users are UM-enabled and can access their mailbox by using Outlook Voice Access. Authenticated users consume Unified Messaging server resources in several ways, including by directly calling in to a subscriber access number, logging on to their mailboxes, accessing their messages, calendar, contacts or the directory, and by using a Unified Messaging server to play voice messages over a telephone using the Play on Phone feature. They can also indirectly consume resources by transferring a call, sending a voice message, or calling a user's extension number and leaving a voice message or sending a fax.
    
  • Unauthenticated   Users who call in to a Unified Messaging auto attendant or call in to a subscriber access number but do not log on to their mailbox are unauthenticated callers. However, Unified Messaging resources are used to service their requests. Every time that a caller calls in to a UM auto attendant or uses a subscriber access number but does not log on to their mailbox, they still consume resources by transferring a call, sending a voice message, transferring to another auto attendant, transferring to another telephone number, or listening to recorded audio prompts.
    

Unified Messaging relies on the ability of the IP gateway to translate TDM or telephony circuit-switched based protocols, such as Integrated Services Digital Network (ISDN) or QSIG, from a PBX to protocols based on VoIP or IP, such as SIP, RTP, or T.38 for real-time facsimile transport.

IP gateways are available from multiple manufacturers in sizes and models that range from 4 ports to 32 ports. You can deploy as many IP gateways as necessary to provide for capacity and fault tolerance. If the number of calls or ports that are required is larger than the number of calls or ports that are supported by a single IP gateway, you can scale horizontally and increase the number of calls that can be accepted or the number of ports by installing and configuring additional IP gateways, creating the UM IP gateway object, and configuring the appropriate hunt groups to support your environment.

It is equally important to match the number of IP gateways that you have in your environment to the number of Unified Messaging servers that are available. For example, you should not configure 10 IP gateways that are each connected with a T-1 line to a single Unified Messaging server. This would mean that the Unified Messaging server would have to support 240 concurrent incoming calls. You should consider this and scale your IP gateways to Unified Messaging servers appropriately.