The Three Responsibilities of Call Routing

The call routing component of CallManager has three main responsibilities:

• To determine which endpoint CallManager should ring based on the digits you dial
• To perform address translation
• To support individualized routing

The first responsibility is to determine which endpoint CallManager should ring based on the digits you dial. These endpoints are often other IP phones, but they could just as easily be numbers controlled by other systems, such as the Public Switched Telephone Network (PSTN), other Private Branch Exchanges (PBX), or other CallManager clusters. Furthermore, the digits you dial can sometimes not even correspond to a physical destination at all. Numbers such as call park codes, Meet-Me conference codes, and translation patterns (the section "Translation Patterns" describes the way to provide aliases for numbers) do not cause any specific device to ring. Rather, they allow CallManager to treat your call in special ways, depending on the type of number. For example, dialing a call park code allows you to retrieve a party who has been held from another station; dialing a Meet-Me conference code allows you to join a multiparty conversation; dialing a translation pattern can redirect your call to a different destination; and dialing a Computer Telephony Interface (CTI) route point can pass control of your call to an application such as an automated attendant. Call routing concepts, such as route patterns, underlie CallManager's treatment of all of these virtual endpoints.

The call routing component's second responsibility is to perform address translation. Address translation allows you to modify the dialed digits and the calling number as the call propagates through a network. Such address translation is important when a network must pass a call from a private network with its private numbering plan to the PSTN with a standardized numbering plan. For example, most PBXs require users to dial an access code to place calls to the PSTN. If CallManager does not first remove the access code before offering the call to the PSTN, the PSTN rejects the call attempt. Imagine what happens if you dial an access code of 9 for calls you make from your home phone; most likely, the PSTN plays an announcement that you have composed your number incorrectly, or worse, it routes your call to a completely different destination. CallManager's address translation capabilities allow you to enforce a private numbering plan while simultaneously reconciling it against the PSTN's numbering plan. The section "Dialing Transformations" discusses address translation in more detail.

CallManager's third responsibility is to support individualized routing, which means that the destination you reach when you dial a number might differ completely from the destination your neighbor reaches when your neighbor dials the same number. This capability is useful to support routing by class of calling user, by organization, or by geographic location. For example, routing by class of calling user permits you to restrict calls made from lobby phones while allowing your executives full access to international numbers. Routing by organization permits you to route calls made by different departments in your enterprise to different locations, so calls from engineers to a technical support organization route to a different place than calls made by marketing executives. Taken to an extreme, routing by organization allows you to control entirely different enterprises using a single CallManager. Routing by geographic location allows you to deploy a single CallManager in one geographic location that controls phones in different geographic locations. Customizing the call routing for users in different geographic locations allows you to deploy multiple sites with an identical number to reach the receptionist: callers in New York reach the New York receptionist when dialing 0; users in Chicago reach the Chicago receptionist when dialing the same number. You can also control costs by routing calls across your IP network instead of the PSTN, a process called toll bypass or toll restriction.