PURPOSE: To exercise securely control over the connection of toll oscillation by making sure of the completion of the dial-tone connection of a pay station and then by supervising dial pulses sent from an extension telephone set to the pay station via an office trunk.

CONSTITUTION: Between the extension telephone set and the pay station to which the telephone set belongs, line circuit LC, switching network SW of PBX, marker MRK, office trunk COT, and oscillating register OR are interposed. After the dial tone of the extension telephone set is received, dialing is done to acquire COT. At this time, OR is being connected. Next, when a toll number is dialed by the extension telephone set, OR releases COT to control oscillation. Consequently, the toll oscillation can securely be controlled.