PURPOSE: To specify a trunk line which has been used when a fault has occurred by providing a signal to be selected output means outputting a signal to be selected showing the channel number of the selected trunk line.

CONSTITUTION: An exchange 2 selects the trunk line 4 based on a dial signal which has been generated from a telephone set 1. Since a dial tone or a ringer signal returns from an opposite station when the trunk line 4 is selected and an exchange in the opposite station completes the preparation for receiving a dial, the signal to be selected showing th channel number of the trunk line 4 is outputted from the signal to be selected output means 5 when the ringer signal is detected. The signal to be selected is transmitted to the exchange 2 on the calling side through a trunk line LINR and is detected by a detector 8. The channel number of the trunk line 4 is stored in a storage part 9 together with the telephone number at colling and called side and is used for searching the fault.