Using a modem involves making and managing a telephone connection, tasks which fall outside most standard communication protocols. To facilitate the use of a telephone connection, Windows CE includes a TAPI service provider for an AT command-based modem, knows as a Unimodem. TAPI is a collection of utilities that allows applications to take advantage of a variety of telephone and communications services without needing detailed knowledge of the particular technology. You can use the TAPI service provider with internal or PC Card modems.
The Windows CE implementation of TAPI focuses on outgoing calls and provides outbound dialing and address translation services. Windows CE does not support inbound calls. To provide flexibility in the choice of hardware, TAPI supports installable service providers.