Abstract: An apparatus is disclosed for monitoring the busy or free condition of telephone subscribers' lines, comprising a number of bridge circuits, each connected to one of the subscribers' lines. Each bridge circuit includes a discriminator which generates a signal when the associated line is busy. In order to achieve a substantial reduction of the transmission loss caused by the bridges, each bridge circuit includes not only a pair of terminals for supplying a voltage compensating the DC voltage on the subscribers' lines, but also a second pair of terminals for supplying a countervoltage to the bridge. The apparatus comprises a device for periodically generating the countervoltage and a device for periodically controlling the countervoltage generator. In order to make the apparatus less sensitive to inphase voltages occurring on the subscribers' lines, the discriminator is connected into the bridge circuit via a low-pass filter.