A319 FLT CREW TAKE CLB CLRNC OF COMPANY FLT WITH SIMILAR FLT NUMBER. AS ZAU AND OTHER COMPANY CLARIFY CLRNC; A319 BEGINS DSCNT TO ASSIGNED ALT.

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ZAU 118.22 CALLED US (ACR X) TO SEE IF WE COULD CLB TO FL350 FOR TFC. CAPT ANSWERED YES; SO ATC ISSUED CLB AND I INITIATED CLB. AT THAT TIME; ACR Y CALLED ATC AND SAID SOMEONE ELSE HAD ANSWERED FOR THEM. ATC IMMEDIATELY ISSUED 'ACR X; MAINTAIN FL330;' AS WE WERE PASSING THROUGH FL340 IN THE CLB. I IMMEDIATELY SET FL330 AND SET FOR 'OPEN DSCNT.' AIRLINES FREQUENTLY USE SIMILAR SOUNDING FLT NUMBERS FOR DEP AND ARR BANKS; MAKING IT EASY FOR MISCOM WITH ATC WITH SIMILAR SOUNDING CALL SIGNS. SUPPLEMENTAL INFO FROM ACN 592864: OUR ORIGINAL FILING WAS FOR FL370 WHICH ZAU WOULD NOT ACCEPT FROM ZMP. BOTH PLTS ARE SURE THAT ZAU SAID 'ACR X' BUT MAYBE WE WERE MISTAKEN. I SAID WE COULD TAKE FL350 OR FL370; OUR ORIGINAL REQUEST. I ACCEPTED FL350 AS COPLT INITIATED AN OPEN CLB TO FL350. SHORTLY AFTER CLB WAS INITIATED; THERE WAS INTERFERENCE ON THE FREQ (118.22) WITH 2 TALKING AT ONCE. I WOULD GUESS THAT ACR Y AND CTR WERE BOTH TALKING. I SUSPECTED WE HAD GOTTEN THE WRONG CLRNC AND MOTIONED THE COPLT TO RETURN TO FL330 AS WE WERE AROUND FL338 BEFORE I COULD GET A WORD IN ON THE FREQ TO TELL CTR THAT WE WERE RETURNING TO FL330. WE CONTINUED ON AT FL330; CLBING TO FL350 AND FL370 SHORTLY AFTER ON ANOTHER FREQ. ANOTHER CASE OF OVERBURDENED FREQ AND SIMILAR CALL SIGNS USED BY THE SAME CARRIER. I AM CERTAIN THAT I READ BACK 'ACR X' IN THE CLRNC TO FL350; BUT HEARD NO CORRECTION ON WHAT WAS A PRETTY BUSY FREQ.