PURPOSE: To search the telephone set originating a malicious call without being noticed by the person making the malicious call by automatically informating the malicious call searching device of a public switch network of the number of the main wire connected the originating telephone set without discontinuing the conversation with the person making the malicious call.

CONSTITUTION: When an incoming call connected to a digital multi-functional telephone set 7 from an arbitrary main wire trunk (COT) 5 through a switch network 4 is malicious call, a prefixed functional button of the telephone set 7 is depressed while the conversation is continued with the person making the malicious call. Upon receiving the automatic malicious call information, a CPU 2 searches a main storage device 3 for the COT 5 connected to a digital line circuit 6 and informs an automatic originating device 8 of the main wire number after finding the main wire number. The device 8 which is actuated by the CPU 2 automatically calls a public switch network 11 and sends the main wire number to a malicious call searching device 9. Thus the originating telephone set can be searched without being noticed by the person making the malicious call.