holding up said DID trunk line from {HEAD ~ said central office} when said extension is busy or does not answer, and terminating {TAIL ~ the call} to said PBX, 
holding up said DID trunk line from { HEAD ~ said central office } when said extension is busy or does not answer , and terminating { TAIL ~ the call } to said PBX ,
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH