the loop between tip 16 and ring 17 by connecting point 41 to point 42 through contacts 37 upon energization of coil 36 it will be understood by those skilled in the state of the art that in a second preferred version of this invention the slight imbalance introduced by having relays 35 and 36 breaking the ring side of the line can be eliminated by breaking the transformer center tap at 27 and inserting the relay circuitry this will provide essentially complete balance to ground in the idle state as well as the operating state the relay comprising coil 38 and contacts 39 is referred to as the supervisory relay and as may be seen from the drawing effects closure across terminal pair 28 upon the excitation of coil 38 in the drawing all relays are shown in their normal position with their respective coils unexcited photodiode 34 is forward biased and thus illuminated when coil 38 is excited therefore photodiode 34 provides a visual indication that the supervisory relay is closed comparator 45 provides an output to point 46 and serves as the driver for the coil 38 of the supervisor relay in the preferred embodiment of the present invention comparator 45 is embodied by a type lm 311 comparator but of course other suitable comparators may be used one of the novel features of the present invention is the unique and simple manner in which a supervisory signal is provided at terminal pair 28 to the pbx equipment as noted hereinabove a short circuit across terminal pair 28 is indicative of a closure of contacts 39 in response to excitation of coil 38 of the supervisory.