SIMILAR SOUNDING FLT NUMBERS AND A HEAVILY ACCENTED DEP CONTROLLER'S VOICE CONTRIBUTE FLT CREW ACCEPTING AND EXECUTING A CLRNC INTENDED FOR ANOTHER ACFT.

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AFTER TKOF; WE WERE ASSIGNED HDG 025 DEGS; 6000 FT. ON THAT VECTOR WE PICKED UP A CLRNC TO TURN TO 335 DEGS AND CLB TO FL140. WE VERIFIED THE CLRNC AND OUR READBACK WAS CONFIRMED 3 TIMES BY ATC. WHILE IN THE TURN AND CLB; WE HEARD ANOTHER ACFT (SIMILAR CALL SIGN) TELL ATC THAT WE WERE PICKING UP 'THEIR' CLRNC TO TURN TO 335 DEGS AND CLB TO FL140. AT THAT POINT; ATC ADVISED US TO RETURN TO 025 DEGS AND 'STOP' OUR CLB AT FL090. WE AGAIN CONFIRMED 'OUR' CLRNC AND COMPLIED. ATC AGAIN VERIFIED OUR READBACK. NO CONFLICT WITH OTHER ACFT. THE PROB HERE IS THAT ATC REFERRED TO US AS ACFT NINE NINE AND ALSO ACFT 'NINETY NINE;' WHICH HAS A SIMILAR RING TO 'NINE TWENTY NINE' WHEN SAID IN HER VOICE. WE PURPOSELY USED 'NINER;' AS PER ICAO GUIDELINES TO ENSURE CLARITY OF OUR CALL SIGN AND 'ARTICULATED' COMS. WITH ATC 'NOT' USING ICAO COMS PHRASEOLOGY; THAT CAUSED A HUGE CONFUSION FACTOR FOR US AND OTHER ACFT; AS WELL AS 'CREATING' A VERY AWKWARD SITUATION IN 'ENDUCING' A FLT DEV OF ALT AND HDG IN A VERY BUSY NETWORK AND ENVIRONMENT AROUND LHR; ALL OF WHICH COULD HAVE BEEN AVOIDED IF SHE HAD ARTICULATED PROPERLY.