FIELD: communications engineering; fighting against commercial and industrial espionage. SUBSTANCE: telephone set mode of operation is determined. In ON-position unauthorized reading of data signals in communication channel is prevented by introducing noise-like signal in analog communication channel or by encrypting data signal in digital communication channel. In OFF-position unauthorized listening is prevented at installation point of telephone set by blocking passage of high-frequency sounding intrusion signals to microphone with aid of low-frequency filter. With digital communication channel telephone set input currents are measured in its both standard modes and in case of unauthorized invocation of telephone set in OFF-position accompanied by increase in input current to value characteristic of ON-position microphone power circuit is broken. Microphone disconnection is recognized by light flashes on front panel of telephone set. EFFECT: enhanced protective efficiency of telephone set including digital communication channel. 7 cl, 1 dwg
