SIMILAR CALL SIGNS CAUSE CONFUSION AT IAH; TX.

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OUR FLT NUMBER WAS ACR X. THE TWR GAVE THE CLRNC FOR ACR Y TO TAXI UP TO AND HOLD SHORT OF RWY 15L. THE FO REPLIED ACR X UP TO AND HOLD SHORT OF RWY 15L. I DID NOT CHALLENGE THE ATC INSTRUCTION; FEELING CERTAIN TWR HAD SIMPLY MADE AN ERROR. WE RECEIVED A CLRNC TO TAXI INTO POS AND HOLD AS ACR X. I AT THAT POINT FELT COMFORTABLE THAT WE WERE IN THE CORRECT SEQUENCE. WHEN TKOF CLRNC WAS RECEIVED I THOUGHT I HEARD ACR X CLRED FOR TKOF. IT WAS NOT UNTIL WE WERE TOLD 'ACR Y CONTACT DEP' I BECAME WARY OF OUR INITIAL TKOF CLRNC. OUR INITIAL CONTACT WITH HOUSTON DEP WE CALLED 'ACR X.' DEP RESPONDED 'ACR Y RADAR CONTACT.' I ASKED THE FO TO RECALL DEP AND EMPHASIZE ACR X NOT Y. DEP SIMPLY REPLIED 'ROGER ACR Y; RADAR CONTACT.' IT WASN'T UNTIL I HEARD ACR Y CHK ONTO ZHU A FEW MINS LATER THAT I FELT CERTAIN AN ERROR HAD OCCURRED. I SPOKE PRIVATELY TO THE CAPT OF ACR Y AND HE SAID HE WAS NOT EVEN IN A POS TO TAXI UP TO AND HOLD SHORT WHEN WE WERE GIVEN OUR INITIAL CLRNC; AND THAT TWR HAD NOT MENTIONED ANY SEQUENCING PROBS TO HIM. ATC NEVER MENTIONED ANY FEELING THAT WE HAD RECEIVED AN IMPROPER TKOF CLRNC EITHER. I FEEL THAT I SHOULD HAVE CHALLENGED SOONER AND RECEIVED A CONFIRMATION OF OUR FLT NUMBER AND SEQUENCING.