A line device is a physical device such as a modem, or an ISDN card connected to a telephone line. Line devices support telephone functions by supporting applications to send or receive data to or from a telephone network. A line device may consist of a set of one or more similar lines used to establish telephone network connections. In TAPI applications, a line device is the logical representation of a physical line device.
TAPI requires that every TAPI-capable line device support Basic Telephony. If an application needs to use capabilities beyond those of Basic Telephony—namely Supplementary Telephony or Extended Telephony—it must first determine the line device capabilities, which can vary according to network configuration (client versus client/server), hardware, service-provider software, and the telephone network. The lineNegotiateAPIVersion function enables the application to identify the line device capabilities. The lineGetDevCaps function returns the telephone device capabilities implemented through the use of the TAPI functions of a specific line device in a data structure of the type LINEDEVCAPS.