Consider a Win32-based application that can make either voice or data (modem) calls. Although these calls could be made simultaneously if a telephone line were in place for each device (the telephone and the modem), assume that there is only one line, so calls are placed one at a time.
For this discussion, a line is defined as a physical telephone line leading from the wall to the telephone company's switch, and a line device (such as a fax or modem) is a local device on the telephone line. Also, this example is restricted to POTS; the telephone line is a standard, two-wire twisted-pair cable that carries an analog signal and constitutes a single channel.
The line devices attached to the computer are visible to the application as instances of the line device class, which is defined by TAPI. The physical telephone is visible to the application as an instance of the phone device class. This application therefore must be able to execute two types of calls: voice and data. One strength of the TAPI programming model is the way its abstraction into classes exploits the similarities between these different types of calls.
For a more in-depth discussion of this process and related ones, see TAPI Applications.