PURPOSE: To obtain test information at the reception telephone terminal equipment not via a test equipment of an exchange by adding a telephone set test request to a user-user information element in a call setting message, sending the resulting information, adding the result of test to the user-user information element in a reply message and returning the resulting message at a reception telephone set terminal.

CONSTITUTION: The central processing unit 10 of a telephone set terminal 12 analyzes a call setting message received via a basic interface circuit 5 and returns a reply message to an ISDN exchange 1 when the result is judged as a test request. When the telephone set terminal 12 finishes the test of the test request items, the unit 10 adds the result of test to a user-user information element in a call interrupt message and the message is sent to the ISDN exchange 1. The ISDN exchange 1 sends the interrupt message added with the result of test to a telephone set terminal 13. The telephone set terminal 13 receiving the interrupt message displays the result of test onto a display device to restore the ISDN line 17.