DC9 CREW ACCEPTED CLRNC FOR OTHER ACFT.

Narrative

FO (RPTR) WAS PF. ON HIGH L DOWNWIND TO RWY 27L AT ATL; WE WERE 9 MI E OF THE RWY WHEN I THOUGHT I HEARD THE CAPT ACCEPT THE CLRNC 'ACR ZYX; TURN L HDG 360 DEGS.' THIS WOULD HAVE BEEN A BASE TURN; AND WAS WHAT WE WERE BOTH EXPECTING (OUR ASSIGNED HDG PRIOR TO THIS WAS 090 DEGS). I WAS HAND FLYING AND BEGAN A TURN TO THE L TO 360 DEGS. AFTER APPROX 10 SECONDS ON THE HDG 360 DEGS; THE APCH CTLR SAID; 'ACR ZYX; YOUR HDG SHOULD BE 090 DEGS; TURN R HDG 100 DEGS; DSND AND MAINTAIN 3500 FT.' I QUICKLY INITIATED THE TURN BACK TO THE R. NO CONFLICT RESULTED; AND THE TCASII NEVER ISSUED ANY TFC WARNINGS. BOTH THE CAPT AND I THOUGHT WE HAD HEARD OUR CALL SIGN; BUT THE CTLR SAID HE HAD BEEN TALKING TO A 'YZX;' NOT 'ZYX.' WE HAVE BOTH ENCOUNTERED AND BEEN SUBJECT TO THIS TYPE OF AUDIO-DYSLEXIA BEFORE. HOWEVER; IN THIS CASE; WE THINK THE CTLR HAD MISSPOKE. CONTRIBUTING FACTORS WERE: 1) THIS WAS THE LAST LEG OF A 4 DAY TRIP; AND BOTH THE CAPT AND FO HAD TIGHT CONNECTING FLT TIMES TO COMMUTE BACK TO THEIR HOMES. BOTH WERE IN A HURRY TO GET TO ATL. 2) IT WAS THE USUAL PLACE FOR A BASE TURN TO OCCUR. 3) THE FREQ WAS VERY BUSY; AND WHEN THE CAPT ACKNOWLEDGED THE CLRNC; HE COULD HAVE BEEN STEPPED ON BY THE PROPER ACFT ACKNOWLEDGING THE CLRNC. 4) SIMILAR CALL SIGNS ON SAME FREQ WITHOUT EITHER CREW BEING TOLD. IN THE FUTURE; WE WILL PAY CLOSER ATTN WHEN OTHER ACFT WITH SIMILAR CALL SIGNS ARE ON THE SAME FREQ. SOMETIMES THE CTLRS WILL BRING THIS TO OUR ATTN; OTHER TIMES THEY WILL NOT.