ZSU CTLR EXPERIENCED APPARENT LOSS OF SEPARATION WHEN RELEASE COORD EFFORTS FAILED WITH NON RADAR FOREIGN TWR CTLR.

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I WORK AT SAN JUAN CERAP IN THE APCH SIDE AND WE PROVIDE SVC TO TORTOLA TWR WHICH IS AN ICAO TWR. THE PROCS BTWN OUR FACILITIES IS NON-RADAR; WHICH MEANS THAT NO ACFT WOULD LAND OR DEPART UNTIL TORTOLA INDICATES THAT THE FIRST ACFT HAS LANDED. ONE IN; ONE OUT RULE BASICALLY; UNLESS THE SECOND PLT SEES THE FIRST ACFT. TORTOLA CALLS ME ASKING IF WE HAD A FLT PLAN FOR ACFT X. I INDICATED YES. I DID NOT RELEASE ACFT X BECAUSE I HAD 2 INBOUNDS. FIRST WAS ACFT Y ESTIMATING AT XA26Z COMING FROM THE E. SECOND WAS ACFT Z ESTIMATING XB34Z; COMING FROM THE S; OUT OF ST CROIX BOTH COORD WITH TORTOLA. ACFT Y SEES THE ARPT AND IS CLRED FOR THE VISUAL APCH AND IS SWITCHED TO THE TWR. ACFT Z SEES ACFT Y AND IS INSTRUCTED TO FOLLOW AND CLRED FOR THE VISUAL APCH. I CALLED TORTOLA AND TOLD HIM THAT ACFT Z IS FOLLOWING THE ACFT Y. TORTOLA SAYS ROGER; THE ACFT X IS NO FACTOR AND SHOW HIM ON THE GND. WHEN HE HANGS UP; AN ACFT X CALLED ME; DEPARTING OUT TORTOLA SQUAWKING XXXX. I ASK HIM IF HE DEPARTED VFR; HE SAYS NEGATIVE; IFR. I TOLD ACFT X TO SQUAWK YYYY; WHICH IS INDICATED IN HIS STRIP. I CALLED TORTOLA AND TOLD HIM THAT I DON'T RECALL RELEASING ACFT X; HE SAYS THAT HE WAS INFORMED THAT ACFT X WAS RELEASED. I CONSIDER THAT THE PROB WAS THE PHONE LINE. WE ARE NOT USING DIRECT LINES; WE ARE USING COMMERCIAL LINES; WHICH CAN BE BUSY; OTS; AND THE TONES ARE LOW. OUR DIRECT LINES HAVE BEEN OTS FOR ALMOST 1 YR. WHEN WE CALL THEM AND WHEN THEY CALL US; THE VOLUME IS SO LOW THAT YOU CANNOT UNDERSTAND THE INFO.