Abstract: A control unit for establishing a four-wire connection between two terminals via a pair of telephone lines comprises switching means which are operable to provide a connection to a pair of telephone lines, signalling means connectible to said lines via said switching means to enable another control unit to be signalled via a line, and line send and receive circuitry connected to the switching means. The line send and receive circuitry has an input for connection to the send channel of a terminal and an output for connection to the receive channel of the terminal. A central control controls operation of the switching means and line send and receive circuitry to automatically establish a four-wire connection and adjust parameters of the line send and receive circuitry to compensate for loss on the telephone line.