ACFT X HAD LTSS FROM ACFT Y. SYS ERROR.

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ACFT X SEBOUND AT FL330 RTE TVC FNT LCK. ACFT Y NWBOUND AT FL350 ON SPACING VECTOR FROM RTE FNT J94 PMM. AN ADDITIONAL 20 ACFT ON FREQ. TFC LEVEL CONSIDERED TO BE COMPLEX. ACFT B WAS ISSUED 'DIRECT PMM' (A L TURN TO APPROX 250 DEG) AND A DSCNT CLRNC TO FL310. X CHKS IN ON FREQ WHILE CTLR IS OFF FREQ. COORDINATING OTHER FLT INFO WITH MY ADJACENT AIRSPACE CTLR. CTLR REALIZES THAT X AND Y ARE A POTENTIAL CONFLICT AND ATTEMPTS TO CONTACT X WITH THE INTENT TO ISSUE A 20 DEG L TURN TO VECTOR X E AND BEHIND Y THAT IS DSNDING AND TURNING W. THE CALL SIGN OF X IS UNFAMILIAR TO CTLR; SO CTLR CALLS X; SPEAKING THE CALL LETTERS PHONETICALLY. RECEIVING NO RESPONSE; AND REALIZING THAT X HAS JUST RECENTLY ENTERED THE SECTOR'S AIRSPACE THE CTLR GOES OFF FREQ TO CONTACT PREVIOUS SECTOR CTLR TO SEE IF X IS STILL IN COM WITH PREVIOUS CTLR. X AGAIN ATTEMPTS TO ESTABLISH INITIAL CONTACT WHILE CTLR IS OFF FREQ BUT THE CALL IS NOT HEARD BY CTLR. THE CTLR AGAIN SPEAKS X CALL SIGN PHONETICALLY TO PREVIOUS CTLR WHO RESPONDS 'I PUT HIM ON YOU; HE'S NOT HERE.' AT THIS TIME THE CONFLICT ALERT ACTIVATES. CTLR ASKS Y TO EXPEDITE RATE OF DSCNT FOR TFC THEN CALLS X 3 TIMES PHONETICALLY BEFORE X PLT RESPONDS. X IS INSTRUCTED TO TURN APPROX 75 DEG TO THE L IMMEDIATELY. Y IS INSTRUCTED TO INCREASE RATE OF DSCNT AND RPT REACHING FL310. X IS CLRED TO CLB TO FL350 AND RPT LEAVING FL335. WHEN THE ACFT WERE 4.687 MI APART THE MODE C ALTS WERE X 331 AND Y 315 RESULTING IN 1600 VERT FT OF SEPARATION. AFTER THOROUGH REVIEW IT WAS DETERMINED THAT THE SPEAKER WHERE RADIO XMISSIONS FROM ACFT ARE ROUTED TO WHEN A CTLR LEAVES THE FREQ TO USE A 'LAND-LINE' TO COORD WITH A CTLR AT ANOTHER SECTOR/FACILITY; (WHICH IS CALLED OPTION 'X') WAS IN FACT DISCONNECTED/BROKEN THUS CTLR WOULD NEVER HAVE HEARD X TRANSMISSIONS. NEARLY 3 MINS ELAPSED FROM THE INITIAL ATTEMPT BY X TO CONTACT THE CTLR AND THE RESULTING LOSS OF SEPARATION. SUGGESTION TO AVOID SIMILAR PROBLEMS. FOREIGN AIR CARRIERS; NEW CARRIERS/ CALL SIGNS OR AIR CARRIER/TAXI OPS ENTERING NEW SVC AREAS HAVE CALL SIGNS NOTED IN REMARKS SECTION OF FLT PLAN. SECONDLY PERHAPS AN INDICATOR LIGHT COULD BE INSTALLED TO INDICATE THAT OPTION 'X' SPEAKER/CIRCUIT IS OPERATIONAL.