ACFT WITH SIMILAR A/N RESPONDED TO ALT CHANGE CLRNC. RPTR QUERIED ATC; TOLD THEY HAD ACCEPTED WRONG CLRNC.

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DURING BUSY DEP; 2 ACFT (FEB AND FB) DEPARTED WITHIN MINS OF ONE ANOTHER. BECAUSE OF HVY LOAD; CTLR WAS ISSUING INSTRUCTIONS IN RAPID FIRE ORDER. FEB AND FB; WHEN SPOKEN VERY QUICKLY; SOUNDED ALMOST IDENTICAL. WE ACCEPTED CLB INSTRUCTION FOR FEB IN ERROR. BOTH PLTS WERE COMPLETELY SURE THAT ALT CHANGE WAS FOR FB. WE HEARD OUR RESPONSE PARTIALLY BLOCKED BY OTHER ACFT. I CALLED DEP CTL TO LET HIM KNOW THAT ANOTHER ACFT HAD ACCEPTED OUR CLRNC. HE INFORMED US THAT WE HAD MADE THE ERRONEOUS ACCEPTANCE BUT NO PROB. BOTH ACFT ON SAME DEP TRACK. SO ALT CHANGE CAME AT NORMAL TIME FOR THAT EVENT; PROBABLY MAKING ANTICIPATION A FACTOR. SINCE THE CLRNC CAME AT THE NORMAL POINT AND TIME; NO CONFLICT RESULTED AS OUR CLRNC FROM 10000 FT TO 12000 FT WAS ABOUT TO BE ISSUED. SINCE BOTH PLTS RESPONDED (PNF ON RADIO AND PF VERBALLY IN COCKPIT); WE WERE BOTH COMPLETELY SURE OF CLRNC. SIMILAR CALL SIGN AND RAPID SPEECH AND ANTICIPATION EQUALED ERRONEOUS ACCEPTANCE. USE OF ICAO STANDARD TERMINOLOGY OF INDIVIDUAL DIGITS MIGHT HAVE PRECLUDED THIS EVENT.