PURPOSE: To secure the safety of privacy by masking an identification number sent from a telephone terminal equipment to an information center.

CONSTITUTION: A caller uses a telephone set 14 to dial an information center 10 and when line connection is made through a telephone network 11, a trunk circuit 22 in the information center 10 detects the arrival of a call and informs it to a central processing unit 32. A voice guide circuit 30 outputs a transmission command of the identification number in voice under the control of the central processing unit 32 and a transmission circuit 28 is connected to the line, from which a disturbing signal by a DTMF signal is generated. The signal is a multi-frequency signal based on the combination of frequencies similar to the multi-frequency of the received identification number and generated and outputted according to a random number pattern in the central processing unit 32. The caller sends the identification number from the telephone set 14 in the DTMF signal. A DTMF reception circuit 26 receives the identification number and when the invasion of the disturbing signal takes place in the telephone network 12, a transmission signal of the transmission circuit 28 eliminates the disturbing signal to implement digital signal processing to the result thereby extracting the identification number. Then the voice guidance circuit 30 sends the information corresponding to the identification number to the telephone set 14 in a voice signal.