TAPI identifies a specific call by means of the call's handle, and TAPI assigns call handles as required. One call handle exists for every call owned or monitored by an application, and an application can obtain call handles in a number of well defined ways. Certain TAPI functions create new calls. As they do so, they return any new call's handle to the application. Sometimes, call handles are provided unsolicited in message sent to the application from TAPI, as is the case with incoming calls or calls being handed off by other applications.
For every call, one handle exists per application—unique call handles are provided to each application by TAPI. This means that different applications with handles to the same call use different handles for it, which limits the scope of a call handle to a single application. In addition, the service provider can assign a unique call ID to a call (unrelated to the call's handle), which is used to track the call across transfers. Whether or not a service provider can assign call IDs to calls is a device capability.
The privileges of an application for a given call are maintained by TAPI and are not the property of an application's handle for the call. (For information about call privileges, see TAPI Applications.) Resources such as memory are allocated dynamically for each call for each application that is given a handle to the call. These resources are not automatically deallocated when the call is dropped as the application may still find it useful to extract information from the call (such as for logging purposes). Therefore, applications must dispose of their call handle when they have finished using it by calling the lineDeallocateCall function.