TAPI (Telephony API) is an application programming interface of the MS Windows operating systems. TAPI enables CTI (computer telephony integration) for Windows applications using common, well-defined interfaces.
TAPI defines 2 interfaces - one for TAPI applications and one for the TAPI service provider (TSP). The TSP provides telephony lines to the TAPI subsystem. These telephony lines will be used by the TAPI application to offer telephony services to the user.
TAPI applications are agnostic to the used telephony technology. This is possible as the TSP abstracts the used technology and implements the TSP interface. Common TAPI applications are the Windows Phone Dialer (dialer.exe, included in Windows) and MS Outlook. Furthermore, many CRM applications support TAPI too.
The TSP maps the technology independent TAPI interface to the respective telephony technology like analog modems, private branch exchanges (PBX) and VoIP systems.
Depending on the used telephony technology a proper TSP must be installed. For example to interact with a PBX usually the TSP of the PBX vendor must be installed. Such TSPs often use a vendor-specific, proprietary protocol to control the PBX. Standardized protocols like CSTA are used rarely. Another popular scenario are analog modems. Analog modems are supported either by using a TSP from the modem vendor or by using the Unimodem TSP included in Windows.
SIPTAPI is a TSP which uses the SIP protocol to interact with a SIP based telephone system. SIPTAPI can interact with a SIP proxy, a SIP PBX or directly with a SIP phone. HTTPTAPI uses HTTP requests to interact with web-enabled telephony platforms and SIP phones with embedded web server.