PURPOSE: To test a subscriber line by a home device by providing an exchange with an audible tone sending device and a bit error measuring instrument.

CONSTITUTION: A telephone set 4 sends a connection request to an exchange 20. A CPU 12 forms a speech path. After automatic incoming to a digital audible tone sending device 8, a subscriber terminal circuit 6 instructs a network terminating device 2 to turn back the used speech path channel by the command of the CPU 12. The terminating device 2 turns back it as it is to form a turning- back loop of 64kbps including a bit error measuring instrument 10. This instrument 10 sends a dummy random digital bit signal to a subscriber line 1 and compares it with the signal turned back by the network terminating device 2 with each bit as the unit to calculate the error rate of the speech path loop. The CPU 12 commands release of turning-back after a prescribed time, and the network terminating device 2 is returned to the normal mode. When the bit error rate is ≥10-3, the instrument 10 sends an audible tone of 103Hz and a maintenance man on the home side finds the wrong state of the laid subscriber line.