Abstract: A telephone set comprising an input unit, a signal generating unit and an information processing unit, which may comprise a read-only memory, a microprocessor or a special LSI circuit for converting information supplied by the input unit into information to be transmitted. In order to obtain a greater reliability as a telephone set, a supervisory circuit, a switching unit and a d.c. isolated signal path have been provided between the switching unit and the information processing unit for coupling the input unit to the signal processing unit via the d.c. isolated signal path and for, possibly, coupling the signal processing unit to the signal generating unit when the supervisory circuit detects proper operation of the information processing unit and for connecting the input unit to the signal processing unit when the supervisory circuit detects a faulty operation of the said information processing unit.