What is TAPI?

TAPI (Telephony API), is the result of  a cooperation between Microsoft and Intel. It works under Windows 95, 98,ME, NT, 2000 and XP. It is wrapped up in a DLL and configurable via the system settings. The TAPI's use is to make an application independent from the hardware. For a TAPI compatible program it does not matter whether the computer has a modem, an ISDN card or another proprietary solution for being connected to the telephone net.

But TAPI is not only offering the user a standardized interface. Towards the hardware side TAPI contains the Telephony Service Provider Interface (TSPI). Hardware producers base their hardware on TSPI.
With TAPI, Microsoft has created a standard for telephony applications. Microsoft is providing the interface with Windows ´95 or later.
No matter what ISDN-application is currently active, they all give their data to the centrally embedded TAPI. It processes the data over the SPI of the hardware provider to the devices.

Architecture:

In general we can distinguish between two kinds of architectures:  First-Party- solutions (TAPI 1.3 to 2.1) and Third-Party- solutions (TAPI 2.1). First Party solutions connect the telephone via the serial interface or a USB port to the local PC. This solution is suitable if the telephone system does not provide an own interface or if only few workstations have to be equipped with a CTI- interface.

With the Third-Party- solution there is no direct link between the local PC and a telephone. Instead, the telephone system is directly connected to the NT Server. If the user wants to dial from the PC, the command is processed to the server. The server will then notifies the telephone system that connects. The workstation telephone rings once and the connection is established..

List of available TAPI drivers for telephones

We also recommend to search for the word TAPI in Google.com