Abstract: A telephone subscriber's line interface for a system in which the telephone instrument and the connections therefrom to the exchange use analogue, while the exchange is digital is described, in which electro-mechanical components are eliminated.To achieve this a microprocessor serves a group of lines on a multiplexed basis, responding to conditions of the line as determined via a high impedance monitor and analogue-to-digital conversion means. It also responds to control inputs from the rest of the exchange. In response to the above inputs it controls line feed, signalling, etc. via digital-to-analogue conversion means and a line driver.