Dial Plan Overview
The dial plan is one of the key elements of a Unified Communications system, and an integral part of all call processing agents. Generally, the dial plan is responsible for instructing the call processing agent on how to route calls. Specifically, the dial plan performs the following main functions:
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Endpoint addressing
Reachability of internal destinations is provided by assigning directory numbers (DNs) to all endpoints.
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Path selection
Depending on the calling device, different paths can be selected to reach the same destination.
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Calling privileges
Different groups of devices can be assigned to different classes of service, by granting or denying access to certain destinations.
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Digit manipulation
Sometimes, it is necessary to manipulate the dialed string before routing the call.
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Call coverage
Special groups of devices can be created to handle incoming calls for a certain service according to different rules (top-down, circular hunt, longest idle, or broadcast).
For general dial plan guidance and design considerations, refer to the Cisco Collaboration Solution Reference Network Design (SRND), available at Link.