FLC RESPONDED TO A CLRNC FOR AN ACFT WITH A SIMILAR CALL SIGN IN ERROR. TRACON FAILED TO NOTE THE WRONG COMPANY CALL SIGN.

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DEN APCH CTL WAS VECTORING US FOR A L DOWNWIND FOR A VISUAL APCH TO RWY 16 AT DEN. WE WERE LEVEL AT 13000 FT MSL; 250 KTS ASSIGNED. COCKPIT WORKLOAD SEEMED NORMAL AT THIS POINT; HOWEVER; WE HAD JUST BEEN GIVEN A RWY CHANGE FROM RWY 35L AND WERE DIGGING OUT OUR APCH PLATES FOR RWY 16. OUR CREW HEARD A CLRNC FOR WHO WE THOUGHT WAS XXX TO DSND TO 9000 FT AND EXPEDITE. I; AS THE PNF; RESPONDED; 'OUT OF ONE-THREE THOUSAND FOR NINER THOUSAND WE'LL HURRY IT UP.' MY RADIO CALL SEEMED CRISP AND CLR WITH NO INDICATION OF A BLOCK BY ANYONE ELSE. PASSING APPROX 11000 FT APCH CTL INSTRUCTED XXX TO SLOW TO 210 KTS. THE FO PF RAISED THE NOSE; EXTENDED SPD BRAKES; SLOWED TO 210 KTS AND CONTINUED DSCNT TO 9000 FT. APCHING 10000 FT; ANOTHER VOICE CAME OVER THE CTL FREQ (POSSIBLY A SUPVR) AND SAID; 'XXX YOU ACCEPTED THE WRONG CLRNC. CLB TO 11000 FT.' I RESPONDED WITH SOMETHING LIKE 'XXX WE'RE AT 10000 FT.' HE TOLD US TO CLB AGAIN SO WE IMMEDIATELY INITIATED A CLB. ABOUT 2 OR 3 SECONDS LATER; WE WERE INSTRUCTED TO DSND TO 8000 FT. WE VERIFIED THAT CLRNC AND BEGAN DSCNT TO 8000 FT. WE WERE THEN CLRED TO TURN BASE AND CHANGE RWYS TO RWY 17R. WE ACCOMPLISHED A HURRIED BUT UNEVENTFUL APCH AND LNDG TO RWY 17R. MY CREW AND I WERE TOTALLY SURPRISED. WE HAD POSSIBLY ACCEPTED THE WRONG CLRNC TO DSND TO 9000 FT. THE CLRNC SEEMED REASONABLE BASED ON OUR POS IN THE PATTERN. WE DID NOT HEAR ANY BLOCKED XMISSION NOR ANYONE ELSE RESPONDING TO APCH CTL. WE WERE TOLD LATER BY A CTL SUPVR THAT THE CLRNC WAS FOR ACR Y. OF COURSE; IT WAS QUITE POSSIBLE I ANSWERED THE WRONG CALL SIGN DUE TO SIMILAR NUMBERS; HOWEVER; NONE OF THE 3 OF US QUESTIONED IT OR NOTICED IT AT THE TIME. NEITHER DID THE CTLR CHALLENGE THE READBACK. ETERNAL VIGILANCE BY ALL CREW MEMBERS WITH EVEN REMOTELY SIMILAR SOUNDING CALL SIGNS IS A MUST; CONSIDERING THE MULTIPLIED THOUSANDS OF RADIO CALLS WE MUST MONITOR AND THE CHANCE OF ERROR INVOLVED. SUPPLEMENTAL INFO FROM ACN 431254: THE BLOCKED READBACK WAS NOT VERIFIED BY ATC. SUPPLEMENTAL INFO FROM ACN 431459: HOWEVER; BOTH THE YYY FLT AND OUR FLT READ IT BACK AT THE SAME TIME. THE ACR Y FLT CALL SIGN COULD BE HEARD AT THE END OF THE TAPE.