PURPOSE: To detect the ring-off and thus to avoid ineffective holding of the unit by providing a detecting means to the central office trunk to detect the current change which is caused when the connection switch is given to the subscriber terminal storing the central office trunk from the talking trunk to the subscriber circuit.

CONSTITUTION: With on-hook of the calling subscriber connected to the telephone exchange side, switching contact X1 and X2 operate through the existing method to give a connection switching to central office trunk COT form relay A within in- coming trunk ICT to relay LR within subscriber circuit LC. Internal resistance 1KΩ of LR is inserted to the side of line A, and the earth is connected to the side of line B. Thus, line A and B are unbalanced. And the potential of intermediate tap S in coil L features a different value from power source E2. As a result, the current flows to mercury relay P for operation of P. Whit operation of P, the ring-off is detected via the contact to release COT.