Call States and Events

A connection is not fully established until both parties are communicating. To reach that point, the establishment of the call goes through several stages, as does the clearing (termination) of the call. A call's events cause it to transition through call states as it comes into existence, is used to exchange information, and terminates. These call-state transitions result from both solicited and unsolicited events. A solicited event is one caused by the application controlling the call (as when it invokes TAPI operations), while unsolicited events are caused by the switch, the telephone network, the user pressing buttons on the local phone, or the actions of the remote party. Some operations on line devices, addresses, and calls may first require that the line, address, or call upon which they operate be in certain specific states.

Different call states indicate that connections exist to different parts of the switch. For example, a dial tone is a particular state of a switch that means the computer is ready to receive digits.

Whenever a call changes state, TAPI reports the new state to the application in a message. This programming model, therefore, is one in which the application reacts to the events reported to it, as opposed to a rigid call-state model. In other words, call-state notification tells the application what the call's new state is, instead of reporting the occurrence of specific events and assuming that the application will be able to deduce the transitions between two states.

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