There are two device classes: line and phone. Each device has a corresponding set of TSPI functions and messages. A line device is any device that has line behavior. A phone is any device that has phone behavior. The service provider implements this behavior by mapping requests into actual device commands.
The SPI can be mapped to a wide range of environments, including those not traditionally thought of as telephony environments. Phone devices and calls on line devices are capable of carrying media streams, such as voice, data, and video.