Abstract: An apparatus for coupling a machine, such as a telephone answering machine, to a telephone line. Included in the apparatus is an optical isolator having a first semiconductor device which, in use, is connected across the line and emits radiation whose intensity represents the level of direct current flowing through the device. A second semiconductor device receives radiation from the first device and generates a current representing the intensity of the radiation. Initially, the apparatus operates in a voltage monitoring mode, wherein the current through the first device represents the level of direct voltage on the line. The apparatus distinguishes therefore between an incoming ringing signal and an outgoing dialling signal. If an incoming ringing signal is detected and the call is not answered by the subscriber lifting his handset, the apparatus operates to connect an associated telephone answering machine to the line and is then switched to a current monitoring mode.