Abstract: The present invention discloses a device for detecting the presence of an intruder listening device on a telephone or data line. The device comprises an electrical circuit electrically connected to the telephone line for measuring the values, polarity and electrical characteristics of the telephone line. These measured values are then compared with predetermined values for each electrical characteristic which are stored by the electrical circuit. An indicator is activated by the electrical circuit when the measured value varies from its respective predetermined value by more than a predetermined amount. The circuit, which is preferably microprocessor controlled, also includes a signal sweep generator for generating one or more electrical wave forms into the telephone line as well as a receiver which receives and analyzes the reflected signal from the telephone line.