Abstract: The apparatus is connected to the mounting cord of a subscriber telephone set and determines whether or not certain clandestine circuits or devices are connected across pairs of conductor lines in the mounting cord. The apparatus generates a pulse voltage and a holding signal which are applied to these lines in order to sustain conduction of breakdown devices, if any, that are connected across the lines. If detectors in the apparatus sense that the line impedance is between predetermined upper and lower limits equivalent to open and short circuits when the holding signal is applied, an indication is made that such circuits or devices are connected across the telephone lines.