ZHU NONRADAR CTLR QUESTIONS B757'S ALT ON A758 STILL IN MMID AIRSPACE WITH OPPOSITE DIRECTION B727; SAME ALT; APCHING MMID FIR. ZHU ISSUES ALERT TO B757 AND MMID AND REMANDS THE B757 BACK TO MMID. THE B757 RPTS 5 MINS LATER AT FL290 AT FIR XFER POINT.

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I WAS WORKING THE RADAR/RADAR ASSOCIATE POS OF THE HOUSTON OCEAN E SECTOR 72R&D. ACFT SWBOUND ON A758 WERE RELAYING NON RADAR POS RPTS VIA NEW YORK COMMERCIAL RADIO. MERIDA E CTLR HAD COORDINATED ACR X; A B757/R; FROM MGGT A758 VIA FRISH INTXN; NEBOUND AT FL290. I HAD COORDINATED ACFT Y; A B722/R; VIA A758 TO MMUN SWBOUND AT FL310. ACFT Y RELAYED TO NEW YORK COMMERCIAL RADIO FL310 ESTIMATE FRISH INTXN AT XA13Z. AT XA12Z; ACFT X CHKED ON MY FREQ ESTIMATING FRISH AT XA18Z AT FL310. I ASKED ACFT X TO VERIFY HIS ALT AND POS. I ISSUED AN URGENT TA AND INFORMED HIM HE SHOULD BE WITH MERIDA ON 128.2 TILL FRISH AND ADVISED ACFT X TO RETURN IMMEDIATELY TO 128.2 FOR CTL INSTRUCTIONS. ACFT X INSISTED MERIDA TOLD HIM TO 'EXPECT FL290;' BUT HAD XFERRED HIM TO MY FREQ. AS I COULD NOT TAKE CTL ACTION IN MERIDA'S AIRSPACE; I TOLD HIM TO CONTACT MERIDA 128.2 IMMEDIATELY. I ADVISED THE MERIDA CTLR OF THE URGENCY VIA THE 358-SHOUT LINE. ACFT WERE ABOUT 3 MINS APART; HEAD-ON; AT THIS TIME! ABOUT 5 MINS LATER; ACFT X CHKED OVER FRISH ON MY FREQ AT FL290. I ADVISED MY SUPVR. HE LATER ADVISED ME THE MERIDA SUPVR HAD SAID ACFT X PLT HAD CHANGED FREQ WITHOUT CTL INSTRUCTIONS. THE PLT INSISTS HE WAS CHANGED TO OUR FREQ. WE ARE TO SWITCH RADIO CTL AT THE FIR. ACFT ARE CHANGING TO OUR FREQ UP TO 10 MINS EARLY AND WE HAVE ADVISED OUR AIRSPACE OFFICE TO CONTACT MERIDA ABOUT THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THIS IS A VERY HEAVILY TRACKED RTE. THE RPTR ADVISES THAT ZHU AND MMID HAVE FREQ AND RESPECTFUL CONVERSATIONS TO ATTEMPT TO RESOLVE ATC PROCEDURAL SITS. THE FAA HAS A XMITTER SITE; CO-USAGE WITH A COMRDO FACILITY AT CANCUN; MEXICO. COM IS GENERALLY ADEQUATE ABOVE FL200; MARGINAL AT THE FIR BOUNDARY BELOW FL280. THE RPTR ADVISED THAT MEXICO IS ATTEMPTING TO PROVIDE FUNDING TO ESTABLISH RADAR COVERAGE THROUGHOUT THE ENTIRETY OF MMID'S CTL. BUT THERE IS A DIFFERENCE BTWN MMID AND ZHU'S 'VIEW' OF AIR TFC EVENTS. AS LONG AS AIR TFC COORD AND COM IS STANDARD; ENGLISH IS USED. NONSTANDARD EVENTS; ATTEMPTING TO VALIDATE CTL SITS; ARE MOST LIKELY RESPONDED TO IN SPANISH -- WHICH MAY OR MAY NOT BE UNDERSTOOD BY THE ZHU CTLR. ZHU CTLRS BELIEVE THAT GOOD EFFORT AND INTENT IS GENERALLY EXERCISED BY MMID CTLRS.