AN ACR MLG ACCEPTS A CLB CLRNC TO FL350 THAT WAS INTENDED FOR ANOTHER ACR FLT.

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WE WERE LEVEL AT FL310 AND EXPERIENCING SOME LIGHT CHOP. I WAS PF AND ASKED CTR (ZBW) FOR FL350 (FO WAS BUSY RETRIEVING AN ITEM FROM HIS FLT BAG). THE CTLR RESPONDED WITH 'ROGER; I HAVE YOUR REQUEST; I'LL GET BACK TO YOU.' BOTH THE FO AND I ACKNOWLEDGED HER XMISSION TO EACH OTHER IN THE COCKPIT. I DON'T REMEMBER IF EITHER ONE OF US RESPONDED ON THE RADIO BUT WE PROBABLY JUST GAVE A 'ROGER.' ABOUT 1 OR 2 MINS LATER; I HEARD (WHAT I THOUGHT WAS) OUR CALL SIGN WITH A CLRNC TO FL350. THE FO LOOKED OVER AND ASKED IF THAT WAS INDEED FOR US AND I RESPONDED IN THE AFFIRMATIVE. THE FO READ BACK; 'ROGER; ACR X-ABYY CLB TO FL350.' IT WAS FOLLOWED BY SILENCE AND WE INITIATED A CLB TO FL350. AT ABOUT FL335 THE CTLR ASKED WHAT OUR ALT WAS AND WE TOLD HER FL335 OR FL337 (SOMEWHERE IN THAT RANGE). SHE THEN TOLD US THAT WE WERE ONLY GIVEN AN 'EXPECT FL350' CLRNC AND GAVE US A 30 DEG TURN TO THE R. SHE THEN CLRED US TO FL350 AND SHORTLY THEREAFTER WE HEARD HER GIVE ACR Y-ZXXY A CLRNC. SIMILAR CALL SIGN; SAME ALTS; ETC. I THINK; IN RETROSPECT; THAT OUR READBACK PROBABLY COINCIDED WITH ACR Y-AXXY'S AND THAT'S WHY IT WASN'T CORRECTED BY THE CTLR. IN THE FUTURE I'LL JUST TAG ON THE WORD 'CONFIRM' WHEN IN DOUBT. I GET SO USED TO ATC CORRECTING ME IF I MAKE AN INCORRECT READBACK THAT IT ENABLED ME TO BE COMPLACENT.