APCH CTLR SWITCHES CALL SIGNS RESULTING IN TCASII RA.

Narrative

I WAS CAPT OF ABC FLT TALKING TO CHICAGO APCH CTL AT 7000 FT; HDG 310 DEGS; SPD 170 KTS ON DOWNWIND TO RWY 14R AT ORD. XYZ FLT WAS SOMEWHERE AHEAD ON DOWNWIND. AT A POINT APPROX 5 MI S OF ORD; APCH SAID 'ABC TURN R TO 050 DEGS; DSND TO 4000 FT.' I REPEATED THE CLRNC AND CALLED OUT OF 7000 FT FOR 4000 FT. AT APPROX 5600 FT IN THE DSCNT ON A 050 DEG HDG; WE RECEIVED A TCASII RA. THE TFC WAS INDICATED AT 2 O'CLOCK; 1 MI AND 400 FT LOW. WE RESPONDED TO THE CLB ADVISORY AND CALLED OUT OUR DIVERSION. APCH SAID 'YOU SHOULD BE AT 7000 FT HDG 310 DEGS.' I REPLIED THAT I HAD READ BACK THE CLRNC DIRECTED TO ABC AND THAT WE HAD A TFC PROB. AFTER LEVELING AT 7000 FT AND A 310 DEG HDG; APCH SAID 'ABC TURN R 050 DEGS.' THIS CLRNC WAS IMMEDIATELY CHALLENGED BY US. APCH SAID 'ABC FLY HDG 310 DEGS; 7000 FT.' APCH THEN GAVE A 050 DEG HDG TO XYZ FLT. I FEEL OUR ORIGINAL CLRNC TO 050 DEGS AND 4000 FT WAS MEANT FOR XYZ FLT. I BELIEVE THIS INCIDENT WAS CAUSED BY: 1) CTLR ERROR. 2) CTLR WORKLOAD. 3) SIMILAR SOUNDING CALL SIGNS; AND 4) MY FAILURE TO IMMEDIATELY QUESTION A CLRNC WHICH 'DIDN'T SEEM RIGHT' FOR ORD. I NOW; ALSO; AM A BELIEVER IN TCASII. SUPPLEMENTAL INFORMATION FROM ACN 249361: IN LATER CONVERSATION WITH CAPT ON XYZ; WE CONFIRMED THAT THE CTLR WAS SWITCHING OUR CALL SIGNS IN ERROR.