ACR FLC ACCEPTED AND INITIATED CLB CLRNC WHICH WAS MEANT FOR ACFT WITH SIMILAR SOUNDING CALL SIGN.

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WHEN CRUISING AT FL310 REQUESTED CLB TO FL350. CTR STATED EXPECT CLB IN A COUPLE OF MINS. A COUPLE OF MINS HEARD A CLRNC TO FL330. THOUGHT IT WAS FOR US AND INITIATED CLB TO FL330. AFTER PASSING FL315 CTLR ASKED IF WE WERE STILL LEVEL FL310. WE SAID NO AND THAT WE WERE CLBING TO FL330. CTLR SAID THAT WE WERE NOT CLRED TO FL330 AND TO RETURN TO FL310. EVEN THOUGH WE READ THE CLRNC BACK THE CTLR SAID THE CLB WAS FOR ANOTHER ACFT WITH A SIMILAR CALL SIGN. THE PROB RESULTED BECAUSE I REQUESTED A CLB; I EXPECTED A CLB AND WHEN I HEARD A CLB I BELIEVED IT WAS FOR ME. ALSO WHEN THE CTLR DID NOT CHALLENGE MY READBACK I KNEW IT WAS FOR ME. I WAS TIRED ON THE FLT BECAUSE OF POOR SLEEP THE NIGHT BEFORE. I WENT TO SLEEP IN ONT AT XA00 AND WOKE UP AT XE30. I TRIED TO SLEEP MORE THAT MORNING BUT NO SUCCESS. WE WERE PICKED UP AT THE HOTEL AND LEFT ONT FOR DEN. AFTER A 2:19 LAYOVER AT DEN; LEFT TO GO TO SEA. THE INCIDENT OCCURRED 15 HRS AFTER I GOT UP THAT MORNING. IF I HAD BEEN RESTED I MAY HAVE ASKED WHY FL330 WHEN I REQUESTED FL350.