AN MD80 FLC HAS DIFFICULTY COMMUNICATING WITH ATC AND RE-ESTABLISHING THEIR CORRECT CALL SIGN WHILE ENCOUNTERING FREQ CONGESTION AT ORD; IL.

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SAFETY CONCERN: COMS SO BUSY ON FREQ THAT ATC COULD NOT RECOGNIZE WE WERE ATTEMPTING TO TELL THEM ABOUT OUR INCORRECT CALL SIGN. ON FIRST CONTACT WITH GND; THEY CALLED US 'XXABC7' WHEN OUR CORRECT CALL SIGN WAS XXABC6. COMPLICATING THE SIT; ATC ISSUED 4 CLRNCS SIMULTANEOUSLY WITHOUT ALLOWING ANY ACFT ACKNOWLEDGEMENT. AS WE TAXIED OUT; ATC AGAIN CALLED US XXABC7 AND AGAIN NO TIME TO RESPOND WAS GIVEN. AN ATTEMPT TO CALL GND AND CORRECT OUR CALL SIGN RESULTED ONLY BLOCK FREQ. WE REALIZED ATC WAS TALKING TO US BECAUSE NO OTHER ACFT FROM OUR CARRIER WAS ACTIVE ON THE FIELD. GND SENT US TO TWR USING ABC7. AGAIN; THE COPLT WAS BUSY WITH PRETKOF DUTIES AND RESPONDED USING (INCORRECTLY) 'ABC7.' I SAID TO CORRECT THIS WITH TWR. WE WERE CLRED ONTO THE RWY AND USED OUR ABC6 CALL SIGN AND THEN IMMEDIATELY CLRED FOR TKOF. WE WERE VERY BUSY AND I'M NOT SURE NOW IF TWR SAID ABC7 OR ABC6; BUT WE RESPONDED XXABC6. THE COPLT SAID WE'D CORRECT IT WITH DEP. WHEN WE CHKED IN WITH DEP WITH XXABC6; THEY INITIALLY SEEMED CONFUSED AND QUESTIONED OUR DEST. HE THEN SAID 'OK; WE'VE GOT IT FIGURED OUT;' AND THE REST WAS UNEVENTFUL. DURING RESEARCH FOR THIS RPT; WE DISCOVERED OUR AIRLINE HAD CANCELED XXABC7 GOING TO ANOTHER DEST; SO THE CONFUSION NOW MAKES MORE SENSE THAN IT DID AT THE TIME. CONCLUSION: WHEN A RED FLAG ARISES; EVEN JUST 1 DIGIT WRONG IN A CALL SIGN; WE NEED TO BE ASSERTIVE IN CORRECTING IT; EVEN IF EVERYTHING ELSE IS NORMAL. ATC THOUGHT WE WERE ANOTHER FLT AND WE DIDN'T REALIZE THIS. THEY FAILED TO NOTE OUR CONTINUED RESPONSES WITH ABC6 OR DIDN'T EVEN PERMIT TIME TO RESPOND. COM MUST BE 2-WAY!