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