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
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
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