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