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