ZOA CTLR DESCRIBED LAND LINE COM PROBLEM WITH ADJACENT INTL FACILITIES.

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I AM AN OCEANIC ATC CTLR AT OAK ARTCC. TODAY WHILE CTLING TFC ON OCEANIC POSITION OC5; I ATTEMPTED TO CALL PORT MORESBY FOR A TRANSFER ON ACR X. I INITIALLY DIALED THE PRIMARY LINE ON THE VSCS PANEL (801 LINE). I RECEIVED A PROMPT ON THE SCREEN THAT ADVISED ME THAT THE LINE WAS 'OTS.' I IMMEDIATELY TRIED THE BACKUP LINE; WHICH REQUIRES A DIAL OF '*5;' FOLLOWED BY THE NUMBER '10.' THIS REPEATEDLY GAVE ME A BUSY SIGNAL. I THEN TRIED THE THIRD OPTION OF PABX; FOLLOWED BY COMMERCIAL TELEPHONE NUMBERS. THESE ALL GAVE ME A BUSY SIGNAL AS WELL. I THEN CALLED THE ADJACENT ATC FACILITY; BRISBANE. I ASKED THEM TO TRY AND REACH MORESBY SO AS TO TRANSFER ACR X. AFTER A FEW MINUTES; BRISBANE ACC CALLED ME BACK AND SAID THEY COULD NOT REACH PORT MORESBY. THIS ALL OCCURRED DURING A BUSY TFC PERIOD; AND COULD HAVE CONTRIBUTED TO AN OPERATIONAL INCIDENT. THIS IS A RECURRING PROBLEM/DISTRACTION IN THE S OCEAN FOR 800; 801; AND 802 LINES (NEW ZEALAND; PORT MORESBY; AND BRISBANE) AND NEEDS TO BE FIXED IMMEDIATELY!!! THIS PROBLEM IS ONGOING AND HAS BEEN RPTED TO LOCAL MANAGEMENT SEVERAL TIMES IN THE PAST. THE LINES REMAIN UNRELIABLE.