Call Control

The developer's view of telephony is one in which telephone lines and phone sets are logically connected through TAPI. This logical connection also provides a point of termination for the telephone line. The physical connection can be made at the desktop, or at a LAN-based host or server, where a LAN protocol extends the connection of the phone lines or phone to the client application. TAPI uses a first-party call-control model on the logically terminated line as well as control of the associated phone device, if any.

Applications access Telephony API services using a first-party call control model. This means that the application controls telephone calls as if it is an endpoint (the initiator or the recipient) of the call. The application can make calls, be notified about inbound calls, answer inbound calls, invoke switch features such as hold, transfer, conference, pickup, and park, and can detect and generate DTMF tones for signaling remote equipment. An application can also use TAPI functions to monitor call-related activities occurring in the system.

In contrast, third-party call control means that the controlling application does not act as an endpoint of the call. A third-party call-control model allows an application to establish or answer a call between any two partiesthe application does not act as either of these parties.

A service provider may implement TAPI's line and phone functions by treating the set of all stations on the switch as a single line device to which multiple phone numbers are assigned. Each phone number on the line device maps to one of the stations on the switchthat is, calls passing through the switch can reach a local station by using its address (telephone number). The application can answer calls or make calls, selecting any one of the addresses on the line device as the origination number. Although the application appears to be the originating party, a call is actually established between the station whose address was selected by its originating number and the other party. However, this implementation is a type of third-party call control and is not a design goal of TAPI, which emphasizes first-party call control applications.

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