the use of a v mos transistor as the semiconductor switching element has the advantage that little energy is required for the control so that a very rapid change over from the non conductive to the conductive state of the saturated semiconductor switching element is possible for short subscriber 39 s lines the energy available between the terminals 1 and 2 is higher than required for a proper operation of the control circuit 5 and the further circuits connected in parallel therewith so that an unnecessarily high line current will flow with all drawbacks this entails such as an excessive dissipation to obviate this drawback the control circuit 5 comprises a current detection device 20 in this embodiment this device is constructed as a voltage stabilising circuit 21 having a threshold device 22 connected thereto instead of a voltage stabiliser 21 it is alternatively possible to use for example a voltage divider the operation of this voltage stabiliser which is known from applicants netherlands patent application no 7307296 is as follows the partial voltage which is produced by the voltage dividers 23 and 24 from the voltage between the connecting point 3 and the terminal 2 is applied to the base of a first transistor 25 the main current path of which is also connected in series with a resistor 26 between the connecting point 3 and the second terminal the base of the transistor 25 is short circuited for alternating current to the terminal 2 via capacitor 27 when the voltage between the terminals 1 and 2 increases slowly the voltage at the base of transistor 25 will increase and this transistor will become more conductive and the line current increases in response to this current increase the voltage drop across the subscriber 39 s.