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Application Message Notification Mechanisms
TAPI 2.0 and above support three mechanisms for notifying applications of
changes in the status of calls, lines, and phones: a callback function, Win32
events, and completion ports. These are described in detail in the documentation for lineInitializeEx and phoneInitializeEx.
Prior to TAPI version 0x0002000, only one such mechanism existed: the callback
function. When the callback mechanism is used, the application's callback
function is invoked from within the application's thread (at the time the
application calls the GetMessage function), providing a normal, fully functional execution environment in
which all Win32 APIs can be safely invoked.
A LINE_REPLY or PHONE_REPLY (asynchronous completion) message sent to the
application carries the request ID and an error indication. Valid error indications
for this reply are identical to those that are returned synchronously for the
associated request, or zero for success. Only the application that issued the
request will receive the reply message, but when the request causes changes in
the state of the device or call, other interested applications may also receive
event-related messages.
TAPI guarantees that a reply message is made for every request that operates
asynchronously, unless the application shuts down TAPI (by calling lineShutdown or phoneShutdown) before the reply is received.
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