PURPOSE: To improve services to subscribers by automatically detecting whether calling is held in readiness or not after answer signal detection.

CONSTITUTION: Once detecting origination, a monitor trunk 1 in a calling state is connected to a receiver 2 through a network 4. A calling-subscriber's number and a called-subscriber's number received by the receiver 2 are stored in a memory provided to a controller 6 and the receiver 2 is released. Once an answer signal is detected, the trunk 1 is connected to a detector 2 through the network 4 and the presence of a call current within a prescribed time is detected. The controller 6 discards the stored information on the trunk having a call current after a prescribed time and marks the stored information on the trunk 1 having no current to report it to a switching system 7 or a charge processing system 8 through an interface part 5.