The TAPI debug binaries can be useful for finding out what is going on behind the scenes in TAPI, and for debugging odd problems, especially if you are trying to debug a Service Provider. You need to install the debug versions of the binaries, enable debugging, and then use a debugger to view messages.

The main way of getting the debug versions of the TAPI binaries is to use the debug/checked builds of the OS, or the debug/checked builds of the Service Pack. If you are part of MSDN, these are easily available to you. The other way is to download the old TAPI 2.1 SDK, which have the binaries for Windows 95 and NT4.

Once the debug versions of the binaries are on your system, to turn debugging on you need to put the following information in the registry.
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Telephony]
"Tapi32DebugLevel"=dword:000000FF
"TapiSrvDebugLevel"=dword:000000FF

This turns on TAPI debugging information. You can use whatever debugger you wish to view the debug strings, I happen to use DebugView. Remember that TAPI32.DLL is running in the context of the application, while the TSP is running in the context of TAPISRV.EXE