Understanding Unified Messaging Hunt Groups

Microsoft Exchange Server 2007 will reach end of support on April 11, 2017. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.


Applies to: Exchange Server 2007, Exchange Server 2007 SP1, Exchange Server 2007 SP2, Exchange Server 2007 SP3

This topic discusses Microsoft Exchange Server 2007 Unified Messaging (UM) hunt groups and how UM hunt groups must be implemented in your Exchange 2007 organization to support Unified Messaging.

What is a Hunt Group?

Hunt group is a term that is used to describe a group of Private Branch eXchange (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 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:

  • Round robin

  • Most idle

  • Start with lowest number

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

Pilot Number

In a telephony network, a PBX or IP PBX can be configured to have a single hunt group or multiple hunt groups. Each hunt group that is created on a PBX or IP PBX must have an associated pilot number. The PBX or IP PBX uses the pilot number to locate the hunt group and in turn to locate the telephone extension number on which the incoming call was received. Without a defined pilot number, the PBX or IP PBX cannot locate where the incoming call was received.

A pilot number is the address or location of the hunt group inside the PBX or IP PBX. A pilot number is generally defined as a "blank" extension number or one extension number from a hunt group of extension numbers that does not have a person or telephone associated with it. For example, you configure a hunt group on a PBX or IP PBX to contain extension numbers 4100, 4101, 4102, 4103, 4104, and 4105. The pilot number for the hunt group is configured as extension 4100. When a call is received on the extension number 4100, the PBX or IP PBX looks for the next available extension number to determine where to deliver the call. In this case, the PBX or IP PBX looks at the extension numbers 4101, 4102, 4103, 4104, and 4105.

Using a pilot number helps eliminate busy signals and helps route incoming calls to the circuits that are available. The PBX or IP PBX pilot number, when it is used with Exchange Server 2007 Unified Messaging, is used as the target. When an incoming call is unanswered or the line is busy, the call is correctly routed to an Exchange Server 2007 Unified Messaging server.

For more information about telephony concepts, see Overview of Telephony Concepts and Components.

UM Hunt Groups

Unified Messaging hunt groups are critical 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. Therefore, a single UM hunt group must be associated with at least one UM IP gateway and one UM dial plan.

Unified Messaging hunt groups are used to locate the PBX or IP PBX hunt group from which the incoming call was 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) signaling message information on the message. The pilot number enables the Unified Messaging server to interpret the call together with the correct dial plan so that the call can be routed correctly. The absence of a hunt group prevents the Unified Messaging server from knowing the origin or location of the incoming call. It is very important to configure the 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 with 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.

For more information about IP gateways, see Understanding Unified Messaging IP Gateways.

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