Understanding Unified Messaging Dial Plans

Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2

Unified Messaging (UM) dial plans are integral to the operation of Microsoft Exchange Server 2010 Unified Messaging and are required to successfully deploy Unified Messaging on your network. The following sections discuss UM dial plans and how UM dial plans are used when you deploy Exchange 2010 Unified Messaging on your network.

Overview of UM Dial Plans

Although Unified Messaging has many Active Directory objects that must be created and configured during deployment, UM dial plan objects are the central component of the Unified Messaging system. A UM dial plan object is an Exchange 2010 organization-wide object created in Active Directory. It represents sets or groupings of Private Branch eXchanges (PBXs) or IP PBXs that share common user extension numbers. In practical terms, all users' extensions hosted on PBXs or IP PBXs within a dial plan contain the same number of digits. Users can dial one another’s telephone extensions without appending a special number to the extension or dialing a full telephone number.

A UM dial plan mirrors a telephony dial plan. A telephony dial plan is configured on PBXs or IP PBXs.

For more information about telephony components, see Understanding Telephony Concepts and Components.

In Unified Messaging, the following UM dial plan topologies can exist:

• A single dial plan that represents a subset of extensions or all extensions for an organization with one PBX or IP PBX.

• A single dial plan that represents a subset of extensions or all extensions for an organization with multiple networked PBXs or IP PBXs.

• Multiple dial plans that represent a subset of extensions or all extensions for an organization with one PBX or IP PBX.

• Multiple dial plans that represent a subset of extensions or all extensions for an organization with multiple PBXs or IP PBXs.

Users who belong to the same dial plan have these characteristics:

• An extension number that uniquely identifies the user mailbox in the dial plan.

• The ability to call or send voice messages to other members in the dial plan using only the extension number.

For more information about how to enable a user for Unified Messaging, see Enable a User for Unified Messaging.

UM dial plans are implemented in Exchange 2010 Unified Messaging to make sure that user telephone extensions are unique. In some telephony networks, multiple PBXs or IP PBxs can exist. In these telephony networks, there could be two users in Active Directory who have the same telephone extension number. UM dial plans resolve this situation. You can put the two users into two separate UM dial plans. This makes their extensions unique.

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

UM dial plans in a single forest in an organization that has multiple physical sites

How Dial Plans Work

When you integrate a telephony network with Exchange 2010 Unified Messaging, there must be a hardware device called an IP gateway that connects your telephony network to your IP-based network. IP gateways convert circuit-switched protocols found in a telephony network to a data-switched protocol such as IP. Each IP gateway in your organization is represented by a UM IP gateway object in Active Directory. For more information about UM IP gateways, see Understanding Unified Messaging IP Gateways.

Exchange 2010 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 an Exchange 2010 computer, 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.

When you create the first UM IP gateway and specify a UM dial plan at the time 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, it 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.

For more information about how to add a Unified Messaging server to a UM dial plan, see Add a UM Server to a Dial Plan.

Types of Dial Plans

A Uniform Resource Identifier (URI) is a string of characters that's used to identify or name a resource. In Unified Messaging, the main purpose of a URI is to enable Voice over IP (VoIP) devices to communicate with other devices using specific protocols. A URI defines the naming and numbering format or scheme used for the calling and called party information contained within a Session Initiation Protocol (SIP) header for an incoming or outgoing call.

The types of UM dial plans you create in Exchange 2010 Unified Messaging will depend on the URI types supported by the IP gateways, PBXs, or IP PBXs in your organization. When you create a dial plan, you should know the specific URI types supported by your PBXs or IP PBXs. There are three formats or URI types that can be configured on UM dial plans:

• Telephone Extension

• SIP URI

• E.164

In Unified Messaging, each time you create a dial plan, the dial plan will, by default, be created to use the telephone extension URI format type. However, you can configure the URI type when you create a dial plan using the New Dial Plan wizard or the New-UMDialPlan cmdlet. After you create a dial plan, you will be unable to change the URI type.

Telephone Extension

The Telephone Extension URI type is the most common type of UM dial plan and is used with PBXs and IP gateways. When you configure a telephone extension (TelExtn) dial plan, the IP gateways and IP PBXs must support the telephone extension (TelExtn) URI type. When the IP gateway or IP PBX communicates with a Unified Messaging server associated with a dial plan, the dial plan must then be configured to support the TelExtn URI type. Generally, most PBXs today support the telephone extension URI type. However, the IP gateway and the UM dial plan must also support the telephone extension URI type.

When a call is received by the PBX or IP PBX and the UM-enabled user isn't available to answer the call, the PBX or IP PBX will forward the call to the IP gateway. In the header for the SIP packet received by the Unified Messaging server from the IP gateway, the calling and called party information will be listed in one of the following formats:

• Tel:512345

• 512345@<IP address>

The telephone extension (TelExtn) format used is based on the configuration of the IP gateway or IP PBX.

SIP URI

Session Initiation Protocol (SIP) is a standard protocol for initiating interactive user sessions that involve multimedia elements such as video, voice, chat, and gaming. SIP is a request-to-response based protocol that answers requests from clients and responses from servers. Clients are identified by SIP URLs. Requests can be sent through any transport protocol, such as UDP or TCP. SIP determines the endpoint to be used for the session by selecting the communication media and media parameters.

When you create a new dial plan, you have the option of creating a SIP URI dial plan that can be used in an environment that has Microsoft Office Communications Server 2007 deployed or in organizations that have IP PBXs. However, in organizations that have IP PBXs, the IP PBXs must also support SIP URIs and SIP routing.

A SIP URI is the SIP addressing scheme used to call another person using SIP. In other words, a SIP URI is a user’s SIP phone number. The SIP URI resembles an e-mail address and is written in the following format: sip:<user name>@<domain or IP address>:Port. When a SIP-enabled IP PBX or an IP gateway is used to send the call to a Unified Messaging server, the device will send only the SIP URI for the calling and called party in the SIP header and will not include extension numbers.

E.164

E.164 is a standard numbering format that defines the international public telecommunication numbering plan used in the Public Switched Telephone Network (PSTN) and some data networks. E.164 defines the format of telephone numbers. E.164 numbers can have a maximum of 15 digits and are usually written with a plus sign (+) before the digits of the telephone number. To dial an E.164-formatted telephone number from a telephone, the appropriate international call prefix must be included in the number dialed. In an E.164 numbering plan for public telephone systems, each assigned number contains a country code (CC), a national destination code (NDC), and a subscriber number (SN).

When you create a new dial plan, you have the option to create an E.164 dial plan. However, if you create and configure an E.164 dial plan, the PBXs and IP PBXs must support E.164 routing. The SIP header received by the Unified Messaging server from an IP gateway associated with an E.164 dial plan will include the E.164-formatted telephone number for the calling and called party information and will be listed in the following format: Tel:+14255550123.

VoIP Security

Unified Messaging servers that have Exchange 2010 installed can communicate with IP gateways, IP PBXs, and other Exchange 2010 computers in either Unsecured, SIP secured, or secured mode, depending on how the UM dial plan is configured. A Unified Messaging server can operate in any mode configured on a dial plan because the Unified Messaging server is configured to listen on TCP port 5060 for unsecured requests and TCP port 5061 for secured requests at the same time. A Unified Messaging server can be associated with a single or multiple UM dial plans and can be associated with dial plans that have different VoIP security settings. A single Unified Messaging server can be associated with dial plans configured to use a combination of unsecured, SIP secured, and secured modes.

By default, when you create a UM dial plan, it will communicate in unsecured mode, and the Unified Messaging servers associated with the UM dial plan will send and receive data from IP gateways, IP PBXs, and other Exchange 2010 computers without using encryption. In unsecured mode, neither the Realtime Transport Protocol (RTP) media channel nor the SIP signaling information will be encrypted. You can use the Get-UMDialPlan cmdlet in the Exchange Management Shell to determine the security setting for a specific UM dial plan.

You can configure a Unified Messaging server to use mutual Transport Layer Security (TLS) to encrypt the SIP and RTP traffic sent and received from other devices and servers. When you add a Unified Messaging server to a UM dial plan and configure the dial plan to use SIP secured, only the SIP signaling traffic will be encrypted, and the RTP media channels will still use TCP, which is not encrypted. However, if you add a Unified Messaging server to a UM dial plan and configure the dial plan to use Secured mode, both the SIP signaling traffic and the RTP media channels are encrypted. An encrypted signaling media channel that uses Secure Realtime Transport Protocol (SRTP) also uses mutual TLS to encrypt the VoIP data.

You can configure the VoIP security mode either when you're creating a new dial plan or after you create a dial plan using the EMC or the Set-UMDialPlan cmdlet in the Shell. When you configure the UM dial plan to use SIP secured or Secured mode, the Unified Messaging servers associated with the UM dial plan will encrypt the SIP signaling traffic or the RTP media channels or both. However, to be able to send encrypted data to and from a Unified Messaging server, you must correctly configure the UM dial plan, and devices such as IP gateways or IP PBXs must support mutual TLS.

For more information about VoIP security and UM dial plans, see Understanding Unified Messaging VoIP Security.

Outlook Voice Access

There are two types of callers who access the Unified Messaging system using the subscriber access number configured on a UM dial plan: unauthenticated callers and authenticated callers. When callers dial the subscriber access number configured on a dial plan, they are considered anonymous or unauthenticated until they input information including their voice mail extension and a PIN. However, the only option available to anonymous or unauthenticated callers is the directory search feature. After callers input their voice mail extension and their PIN, they will be authenticated and given access to their mailbox. After they gain access to the system, they are using the Outlook Voice Access feature. Outlook Voice Access is a series of voice prompts that give the caller access to e-mail, voice mail, calendar, and other information. Outlook Voice Access lets authenticated callers navigate their personal information in their mailbox, place calls, or locate users using dual tone multi-frequency (DTMF), also known as touchtone, inputs or voice inputs.

Subscriber Access Numbers

After you've created a UM dial plan, you need to add at least one subscriber access number. Subscriber access numbers are also called pilot numbers. This number is used by Outlook Voice Access users to access their mailboxes and lets them search the directory.

By default, when you create a UM dial plan, no subscriber access number is configured. To enable subscriber access, you must configure at least one telephone or extension number. The number of alphanumeric characters in the subscriber access number can't exceed 20. After you configure this number on the dial plan, this number will be displayed in the Microsoft Office Outlook 2007, Outlook 2010, and Outlook Web App voice mail options.

You can use the Enter the telephone number to associate field on the UM dial plan to add a telephone number or extension that a user will call to access the Unified Messaging system using Outlook Voice Access. In most cases, you'll enter an extension number or an external telephone number. However, because this field accepts alphanumeric characters, a SIP URI can be used if you're using an IP PBX.

Depending on the needs of your organization, you may need one or more subscriber access numbers. You can have a single subscriber access number configured on a single UM dial plan or you can have multiple subscriber access number in a single UM dial plan, but you can't have a single subscriber access number that spans multiple UM dial plans.

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