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