TSPI allows you to develop back-end services that handle requests from applications that conform to the Windows Telephony API.
A Telephony Service Provider Interface (SPI) is implemented by telephony service vendors who link the software code (DLLs, device drivers, and firmware) and hardware (add-on hardware, phone sets, switches, and networks) that jointly implement the Telephony SPI.
The type of hardware devices that can be supported include:
ISDN is a digital telephone service that works over existing copper telephone wiring.
A PBX manages call traffic between an organization's phones, within and outside the company.
POTS provides functionality such as make call, hang up, and answer call.