Telephony Application Programming Interfaces

http://msdn.microsoft.com/en-us/library/windows/desktop/ms734257(v=vs.85).aspx 

http://msdn.microsoft.com/en-us/library/ms734273(v=vs.85) 

http://www.codeproject.com/Articles/10994/TAPI-3-0-Application-development-using-C-NET