Abstract: The present invention provides an apparatus for selectively connecting one of a plurality of data units (21,22,23) (for example modems) to a telephone line (20). The apparatus comprises a plurality of interface circuits (24) each having a respective data unit connected thereto. Also connected to each interface unit, and thereby connecting the interface units together, is a common dialing device (28), a telephone line, a dial request bus and a line occupied bus. Each interface circuit has a dial connect switch and a line connect switch, a first switching means for connecting the telephone line to the common dialing device, on operation of the dial connect switch, and a second switching means for disconnecting the telephone line from the common dialing device and connecting the telephone line to a data unit on operation of the line connect switch.