MLG FLC ACKNOWLEDGED CLB CLRNC ONLY TO BE DISPUTED BY ZAU CTLR. CTLR XMITTING ON MULTIPLE FREQS WITH SIMILAR SOUNDING CALL SIGN.

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ACR A FLT WAS AT LEVEL FLT AT FL250 WHEN ATC ASKED US IF WE COULD BE LEVEL AT FL290 IN 1 1/2 MINS. ATC CALLED US AS ACR A 123. WE ACCEPTED THE CLRNC WITH THE RESTRS. PASSING ABOUT FL275 WE WERE TOLD BY ATC TO IMMEDIATELY DSND TO FL240. WE COMPLIED AND THEN WERE TOLD BY ATC THAT THERE WAS ACR B 123 ON THE FREQ. WE NEVER HEARD ACR B 123 ON THE FREQ. ATC INDICATED THAT HE PROBABLY SAID 'ACR A' WHEN HE MEANT 'ACR B.' ATC WAS WORKING MORE THAN 1 FREQ AT THE TIME. WE ALSO WERE NEVER ADVISED ABOUT ACR B 123 BEING AROUND. I DON'T BELIEVE THERE WAS A TFC CONFLICT LATER ON WITH ORD APCH THERE AGAIN WAS SOME CONFUSION WITH OUR CALL SIGNS. AGAIN ACR B 123 WAS ON A DIFFERENT FREQ THAT WE COULDN'T HEAR. BOTH TIMES ATC WAS SURPRISED ABOUT THE CONFUSION. FREQ CONGESTION WAS NOT NECESSARILY THAT BUSY; AT LEAST THE ONES WE WERE LISTENING TO. THERE WAS A COMMENT ON THE RADIO BY ATC THAT HE WAS VERY BUSY WITH 'ALL' THE FREQS HE WAS WORKING. THE SOLUTION TO THIS PROB IS VERY SIMPLE -- CTLRS SHOULDN'T OPERATE MORE THAN 1 FREQ AT A TIME.