PURPOSE: To store contents contained in an incomplete call setting message and to display them to an operator by providing a monitoring circuit, a message identifying and deciding circuit composed of a message deciding means, etc., a storage circuit, etc.

CONSTITUTION: A layer 1 terminating circuit 1 is connected through a communication connector S for an integrated service digital network(ISDN) to the S point of the ISDN terminal, and the circuit 1 terminates a layer 1. Next, a layers 2 and 3 monitoring circuit 2 monitors a call control signal on the prescribed channel of a user.network interface, and the circuit 2 extracts the signal contents of a layer 3. A message identifying and deciding circuit 3 identifies the call setting message and a response confirming message in a layer 3 message from a network side, and when the response confirming message to have the same call number as the call number in the call setting message is not returned within a prescribed time, the circuit 3 decides that the reception is incomplete. Thus, the contents contained in the call setting message to be made incomplete can be displayed through a storage circuit 4 and a displaying circuit 5 according to the request of the operator.