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