A DC9-30 FLC TAKES THE CLRNC FOR ANOTHER ACR AND EXPERIENCES A HDG ALTDEV WHILE GOING INTO STL; MO.

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FLT ACR X; XYZ; WAS DSNDING TO 7000 FT MSL ON RIVERS 2 ARR. AT 13 DME FROM CARDINAL VOR; 1 MI PRIOR TO MELVY ON ARR; CAPT ASKED COPLT TO CONFIRM ASSIGNED HDG AFTER MELVY. CAPT WAS FLYING AND COPLT WAS WORKING RADIO. APCH CTL RESPONDED THAT HDG FOR ACR X; FLT XYZ; WAS 125 DEGS AFTER MELVY. FLC THEN THOUGHT THEY HEARD APCH CTL SAY 'ACR X; XYZ; YOU ARE COMING DOWN PRETTY GOOD; SO COME R TO 210 DEGS AND DSND TO 2500 FT.' ACR X; FLT XYZ; WAS INDEED DSNDING RAPIDLY; SO WE BELIEVED THE CALL WAS FOR US. COPLT READ BACK 'R TO 210 DEGS; DSND TO 2500 FT; XYZ.' WHILE IN TURN; APCH CTL STATED 'ACR X; FLT XYZ; WAS SUPPOSED TO HAVE TURNED AT MELVY TO 125 DEGS AND MAINTAIN 7000 FT.' APCH CTL ASSIGNED A HDG AND CAPT STOPPED DSCNT AT APPROX 5500 FT AND RETURNED TO 7000 FT. CAPT ASKED FOR APCH CTL TO PULL AUDIO TAPE. APCH CTL PROVIDED FURTHER VECTORING. REST OF APCH AND LNDG UNEVENTFUL. AFTER LNDG; CAPT CALLED APCH CTL SUPVR. SUPVR STATED THAT HE LISTENED TO TAPE SEVERAL TIMES. SUPVR STATED THAT THE TAPE PRESENTED THE FOLLOWING: CTLR RESTATED 'ACR X; XYZ; HDG AFTER MELVY WAS 125 DEGS.' CTLR THEN STATED ALMOST IMMEDIATELY 'ACR Y; XYZ; YOU ARE DSNDING PRETTY GOOD SO TURN 210 DEGS AND DSND TO 2500 FT.' CTLR WAS WORKING ACR Y; XYY; BUT USED 'ACR Y; XYZ.' FLC RESPONDED TO 'XYZ' CALL WITH FOLLOWING RESPONSE: 'R 210 DEGS; DSND TO 2500 FT; XYZ.' CREW DID NOT USE CALL SIGN 'ACR X.' CTLR WAS VERY QUICK TO NOTE TURN. SUPVR STATED ON PHONE THAT NO PAPERWORK WOULD BE FILED AND CONSIDERED IT A LEARNING EXPERIENCE FOR FLC AND CTLR. SUPVR CAUTIONED FLC TO USE CALL SIGN CONSISTENTLY.