TAPI stands for Telephony Application Programming Interface and is a specification for programming telephony equipment.  The most widely used TAPI device is probably the data modem, but modems support only a subset of the capabilities that TAPI can control.  Products that provide more complete support include TAPI-enabled telephones, PBXs, telephony boards, and more.

Windows has included TAPI support since Windows 95.  The advantage of TAPI for hardware manufacturers is that TAPI-compliant hardware can be used with almost any TAPI-compliant software.  Similarly, TAPI-compliant software can be used with almost any TAPI-compliant hardware.  An advantage to Windows users is they can use more telephony hardware and software and, more significantly, their modems can be shared by multiple programs at the same time.  This does not mean you can connect to the internet and receive a fax at the same time, but it does mean you don't have to quit your fax program every time you dial up your Internet Service Provider, as the case used to be.

Ascendis Caller ID normally uses TAPI to communicate with your modem or other TAPI device.  So, Ascendis Caller ID can be left running at all times, even while you connect to the internet, or send or receive faxes.

TAPI is not without disadvantages.  It is a large, complex specification that makes it harder, initially, to control a modem.  Also, many, many modem manufacturers make mistakes or omissions when specifying the features of their modems.  This is usually the reason why a modem with caller id capabilities does not work with Ascendis Caller ID or other TAPI applications.  Ascendis Caller ID's Repair Modem feature can repair some of these configuration errors.

Ascendis Caller ID can access modems using TAPI or directly, as specified in the Device Properties window.