Abstract: Method and apparatus for detecting whether or not an unauthorized surveillance device has been installed on a communications line between user terminal equipment and central switching station equipment. Apparatus is installed at the central switching station and receives a test request signal from the user terminal equipment, such as a telephone. A receiving device receives the remotely generated test signal and activates the test. A measuring device then measures electrical characteristics of the communications line to determine the presence or absence of the "bug". The measuring device provides output signals indicative of the security status of the line. A signalling device responds to the output signals and provides a status signal to the user terminal equipment through the communications line. The status signal indicates the security status of the communications line.