To test at least one subscriber line that can be connected to a line unit, at least one test signal is emitted along the subscriber line, when the latter is not actively connected, and at least one resultant first echo signal is received, recorded and stored. In the short-circuited or open condition of the subscriber line, the test signal or signals is/are emitted via said line and at least one resultant second echo signal is stored. If an error occurs, the test signal or signals is/are emitted along the subscriber line and at least one resultant error echo signal is recorded, said error echo signal being compared with the first and/or second echo signal or signals. The advantage of the invention is that the location of an interruption in the subscriber line can be determined locally without the use of complex measuring devices.