ACR PIC CHALLENGES ALT CHANGE ASSUMPTION WITH MTEG ACC.

Narrative

INCIDENT OCCURRED ON REGULARLY SCHEDULED FLT FROM ARUBA TO NYC-JFK. AT CRO (CABO ROJO) VOR; A FIR XING BTWN SANTO DOMINGO CTL (DOMINICAN REPUBLIC) AND PORT-AU-PRICE CTL (HAITI); I MADE A POS RPT AT CRO AND FL310. PORT-AU-PRINCE ACKNOWLEDGED THE POS RPT; AND BOTH THE CAPT (PF) AND I THOUGHT WE HEARD 'CLB TO FL350; RPT JOSES (INTXN/FIR BOUNDARY);' WHICH I REPEATED BACK TO CTLR. WE STARTED A CLB; AND AT FL320 THE CAPT HAD ME ASK THE CTLR TO REPEAT THE ALT CLRNC. WE THEN RADIOED HE MEANT US TO STAY AT FL310; AND EXPECT FL350 LATER. WE IMMEDIATELY DSNDED BACK TO FL310. FLT CONDITIONS WERE DUSK (SUNSET) AND GOOD VISIBILITY. WE SAW NO OTHER TFC VISUALLY OR ON OUR TCASII PRESENTATION. FACTORS CONTRIBUTING TO THIS INCIDENT INCLUDED THE CTLR'S EXTREMELY THICK ACCENT (VERY HARD TO UNDERSTAND); THE NON RADAR ENVIRONMENT OF THE AIRSPACE OVER HAITI; AND HIS PHRASEOLOGY FOR A LATER; TO-BE-ASSIGNED ALT. NEITHER THE CAPT NOR I HEARD 'EXPECT FL350' WHICH IS OUR USUAL CLUE THAT IT IS NOT THE CURRENT CLRNC. IN THE FUTURE; I WILL REDOUBLE MY EFFORTS TO SEPARATE THE CURRENT ATC CLRNC FROM A FUTURE CLRNC; REGARDLESS OF THE CTLR'S ACCENT OR PHRASEOLOGY. AND; OF COURSE; IF THERE IS ANY DOUBT; HAVE THEM REPEAT IT UNTIL IT IS UNDERSTOOD BY ALL PARTIES.