Abstract: The disclosed signal multiplexing circuit makes use of a single pair of wires to interface a data and voice communication circuit such as an electronic key telephone (EKT) station set with a central data and voice communication system such as a business communication system (BCS). The signal multiplexing circuit transmits outgoing standard voice signals, outgoing data and outgoing auxiliary signals and, in turn, simultaneously receives incoming standard voice signals, incoming data and incoming auxiliary signals on the single pair of wires. Both time division and frequency separation techniques are concurrently employed to achieve multiple use of the single pair of wires without crosstalk between the plurality of signals. The data signals are transmitted as bipolar bit pairs preceded by a data start pulse pair. The transmission of an outgoing data bipolar pulse pair is delayed until the incoming pulse pair has cleared the line, thus separating incoming and outgoing data in time.