Abstract: In a telephone system supervision is provided for, between a local terminal typically associated with a telephone switching facility and a remote key telephone terminal. A data link between the terminals includes a two wire transmission line terminated with hybrid circuitry. The hybrids provide full duplex operation. Electronic circuitry, in the local and remote terminals, operates to provide continuous two way transmission of supervisory data by means of diphase signals. A coded diphase signal transmitted from the local terminal has a baud rate twice that of the conventional diphase signal transmitted back from the remote terminal. This allows simple receiver sync extraction circuitry in both the remote and local terminals. The length of transmission line permissible between terminals is limited by a maximum permissible one way delay of half the period of the maximum baud rate.