B777 FLT CREW EXPERIENCES COM DIFFICULTY WITH OCEANIC CTL ON MIA -- LHR RTE.

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APCHING OYSTER AT FL370; WE EXPERIENCED DIFFICULTY MAINTAINING VHF AND HF COM WITH MONCTON AND GANDER OCEANIC. NO CLB CLRNC WAS HEARD UNTIL APPROX X005Z WHEN WE CLBED AS INSTRUCTED TO FL380. AGAIN POOR HF CAUSED A 2-3 MIN DELAY IN GANDER ACKNOWLEDGING OUR BEING LEVEL. WE WERE NEVER NO COM JUST EXPERIENCING POOR PROPAGATION AND FREQ CONGESTION. SOME 30 MINS LATER; WE RECEIVED AN INQUIRY FROM GANDER. THIS WAS IMPOSSIBLE TO COMPLETE WITH THE COM CONDITIONS; AND WE CALLED THEM ON SATCOM. REPEATED CALL CALLS WERE RECEIVED FROM GANDER DURING THIS EPISODE. WE COULD HEAR THEM WITH POOR TO ACCEPTABLE RECEPTION. THEY COULD NOT HEAR US. SUPPLEMENTAL INFO FROM ACN 595546: UPON RETURNING FROM MY REST BREAK; THE FO AND RELIEF PLT SAID THAT THEY HAD A SMALL COM PROB WITH GANDER OCEANIC. WHEN WE CROSSED OYSTER ONTO TRACK X; THERE WAS A LOT OF RADIO TFC AND THEY WERE NOT ABLE TO TALK TO GANDER. THEY HAD RECEIVED THEIR OCEANIC CLRNC TO CROSS OYSTER AT FL370 WITH 'EXPECT FL380.' THEY COULD NOT COM WITH GANDER UNTIL A FEW MINS LATER WHEN THEY THEN CLBED TO FL380. WHEN I GOT BACK INTO THE COCKPIT; THE ISSUE SEEMED RESOLVED AND NO OTHER ACTIONS WERE NECESSARY.