The TAPI dynamic-link library allocates memory for each call for each application that has a handle to the call. It is likely that service providers will allocate memory to hold call information as well. Deallocation of an application's call handle allows the library and the service provider to reclaim these memory resources. An application's handle for a call becomes void after a successful deallocation.
An application's attempt to deallocate the handle of a non-idle call for which it is the only owner will fail. The application should either first hand off ownership and change its privilege to monitor, or simply try to change its privilege to monitor (in case there are other owners) or clear the call by dropping it, which places the call into the idle state, and then deallocate its handle.