A HOLDING CRJ200 STARTS A DSCNT MEANT FOR ANOTHER ACR FLT WHEN THE PIC ANSWERS TO THE WRONG CALL SIGN WHILE HOLDING SW OF ORD; IL.

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FOR ATC PURPOSES; OUR CALL SIGN IS LIMITED TO THE LAST 3 NUMBERS (IE; ACR XXX). BECAUSE OF TSTMS IN AND AROUND CHICAGO; WE HAD BEEN HOLDING AT BRADFORD ON THE BRADFORD ARR INTO CHICAGO. ALTHOUGH 'HOLDING;' MY WORKLOAD AS THE PNF WAS HIGH. BECAUSE OF FUEL CONSIDERATIONS; I HAD BEEN LISTENING TO ATIS AT PIA; SPI; BMI; AND MLI. FURTHERMORE; I HAD BEEN ATTEMPTING TO REACH COMPANY DISPATCH ON GND/AIR FREQ AND COMMON RADIO; FINALLY SUCCEEDING ON THAT FACILITY. WHILE I WAS ATTENDING TO THESE DUTIES; THE CAPT HAD ASSUMED COM #1 WHILE FLYING THE AIRPLANE. WE WERE CLRED DIRECT ORD AND MAINTAIN 6000 FT. I ACKNOWLEDGED THE CAPT EXITING THE HOLD AND SETTING THE ALT SELECTOR TO 6000 FT. I DID NOT ACTUALLY HEAR THE ATC CLRNC. EVERYTHING SO FAR WAS OK. WITH THE HVY WORKLOAD OF OUR CREW; THE APCH CTLR; THE WX; AND THE ARR OF ANOTHER ACR FLT YXX ON THE APCH FREQ; A BREAKDOWN OF COM BEGAN TO OCCUR. ATC BEGAN CALLING US; 'ACR XXX; ACR YXX;' AND 'OTHER ACR YXX AS OTHER ACR XXX.' ATC HAD NOW CLRED 'OTHER ACR XXX' TO 4000 FT. THE CTLR MEANT FOR 'OTHER ACR YXX' TO DSND TO 4000 FT. MY CAPT HEARD '...XXX' AND DIALED IN 4000 FT; I ACKNOWLEDGED (NOT HAVING ATC BECAUSE OF GARBLE ON COM #2) AND WE BEGAN TO DSND TO 4000 FT. THE CREW OF 'OTHER ACR YXX' WAS THE FIRST TO PICK UP ON THE GROWING CONFUSION. HE QUICKLY QUERIED ATC TO CLARIFY THE LAST CLRNC. MY CAPT ARRESTED THE DSCNT IMMEDIATELY AND CLBED BACK TO 6000 FT. EVERYTHING HAPPENED QUICKLY EXCEPT OUR DSCNT; WHICH REVEALED A TOTAL ALTDEV OF ABOUT -300 FT. ONCE BACK AT ALT; ATC SAID 'I GUESS YOU HAVE DISCOVERED THAT CLRNC WASN'T FOR YOU.' THE TONE WAS ONE THAT SIMPLY STATED; BOTH MY CREW AND ATC ERRED EQUALLY. 'OTHER ACR YXX' SAVED THE DAY. THE AIRSPACE AROUND CHICAGO WAS NOT CONGESTED BECAUSE OF GND HOLDS AT ORD AND MDW DUE TO WX. AT NO TIME WAS THE SAFETY OF ANY ACFT AT RISK. FINALLY; A SOLUTION COULD BE ACARS WHICH COULD HAVE SPED UP MY DUTIES ON COM #2 AND WOULD HAVE ALLOWED ME TO LISTEN TO XMISSIONS ON COM #1. ACARS DOES NOT EXIST ON OUR ACFT FLEET.