The apparatus further includes a switching network for coupling {HEAD ~ the door speaker} and microphone to {TAIL ~ a local telephone set} and a remote telephone set selectively in any combination 
The apparatus further includes a switching network for coupling { HEAD ~ the door speaker } and microphone to { TAIL ~ a local telephone set } and a remote telephone set selectively in any combination
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH