External Call Routing

When you place a call from a Cisco IP Phone, Cisco CallManager analyzes the dialed digits. If the dialed number matches a directory number (DN) that is registered with the Cisco CallManager cluster, Cisco CallManager routes the call to the destination Cisco IP Phone that is associated with the matching DN. This type of call is an internal (or on-cluster) call. Cisco CallManager handles the call internally without the need to route the call to an external PSTN gateway.

IP Phones are not the only devices that can place and receive internal calls; any device that registers a DN with Cisco CallManager can place and receive internal calls. Examples of other devices include the Cisco IP SoftPhone/IP Communicator and analog telephones that are attached to Media Controller Gateway Protocol (MGCP) or Skinny Client Control Protocol (SCCP, or Skinny)based gateways.

When a Cisco IP Phone dials a number that does not match a registered DN, it assumes that the call is an external (or off-cluster) call. Cisco CallManager then searches its external route table to determine where to route the call. Cisco CallManager uses the concept of route pattern and translation pattern tables to determine where and how to route an external call. The route pattern and translation pattern tables are very similar to the routing table that a Cisco router maintains for routing data.

You can create external route plans based on a three-tiered architecture that allows multiple layers of call routing redundancy as well as digit manipulation. Route patterns match external dial strings, in which a corresponding route list will select available paths for the outbound call based on priority. Cisco refers to these paths as route groups, which are very similar to the trunk group concept in traditional PBX terminology. You can think of a route pattern as a static route with multiple paths that you can prioritize. Figure 10-1 depicts the three-tiered route plan architecture.

Figure 10-1. Cisco CallManager Route Plan Architecture

In addition to facilitating multiple prioritized paths for a given dialed number, the route plan can also provide unique digit manipulation for each path, based on the external network requirements. Digit manipulation involves adding or subtracting digits from the original dialed number to accommodate user dial habits and to ensure that the external network or PSTN receives the correct digits to place a call.

Even though the CallManager processes these route plans from the top down (a user dials the route pattern, which directs the call to a route list, then to the preferred route group, and finally to the device), the configuration of the route plan occurs from the bottom up (devices are added, route groups are created from the devices, route lists are created from the route groups, and the pattern points to the list).