NMAC. FOREIGN AIRSPACE ACFT CLRED THROUGH ANOTHER'S ALT.

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IN CONTACT WITH YANGON CTL AT FL350. REQUESTED FL390. CHKED DWI AND CLRED TO FL390 BY YANGON ON 134.2. AT APPROX XXXX MADE VISUAL CONTACT WITH ACR X. HE WAS LEVEL AT FL370 EBOUND; WE WERE CLBING OUT OF FL365 AT ABOUT 1000 FT. HE WAS TO OUR R. WE CONTINUED THE CLB WITHOUT MAKING ANY MANEUVER MAINTAINING VISUAL SEPARATION. ACR X MENTIONED THAT HE HAD PICKED US OUT ON HIS TCASII AND THAT HE WAS FILING A NEAR MISS. WE HAD NOT HEARD ACR X ON YANGON 134.2 FREQ SINCE WE HAD ONLY BEEN ON THE FREQ FOR A FEW MINS AND HE HAD NOT MADE A RPT. IT APPEARS HE DID NOT HEAR US GIVEN A CLRNC WHICH CLBED US THROUGH HIS LEVEL. HE WAS AS SURPRISED AS WE WERE BY THIS SUDDEN ENCOUNTER. I CAN ONLY SURMISE THAT THE CLRNC GIVEN TO US BY YANGON CTL WAS A PLAIN AND SIMPLE MISTAKE. I CAN SEE NO OTHER POSSIBLE FACTORS. I HAVE NOT FLOWN THIS SECTOR MORE THAN HALF A DOZEN TIMES; BUT THIS SECTION OF AIRWAY IS ALWAYS BUSY WITH ACFT CLBING OUT OF BANGKOK AND ACFT WANTING TO START THEIR DSCNT INTO BANGKOK. IT WOULD CERTAINLY BE SAFER I FEEL; IF 1 CTL CTR COVERED SAY 250 NM FROM BANGKOK; WE WOULD AT LEAST HAVE HEARD ACR X. AS IT IS; THE FREQ CHANGEOVER OCCURS AT A CRITICAL GEOGRAPHICAL POS. INCIDENTALLY; HE HAD CONTACTED YANGON CTR BEFORE ENTERING MYANMAR AIRSPACE; ALTHOUGH I DON'T BELIEVE THIS HAS ANY PARTICULAR RELEVANCE.