AN ACR LEAVES ITS ASSIGNED ALT WHEN IT TAKES A CLRNC MEANT FOR ANOTHER COMPANY ACFT WITH A SIMILAR CALL SIGN WHO WAS NOT ON THE SAME FREQ.

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WE WERE CRUISING AT FL310; DIRECT TO APPLETON VOR. WE WERE TOLD TO DSND TO FL290; AND WE COMPLIED. A FEW MINS LATER; WHILE WE WERE CRUISING AT FL290; ZID APPARENTLY DIRECTED 'ACR X ABCD; DSND TO FL270' (FROM FL290). I MISUNDERSTOOD THE CALL SIGN AND ACKNOWLEDGED 'ACR X AECF OUT OF FL290 FOR FL270.' THE CTLR DID NOT CATCH THE ERROR; AND BOTH OF THE PLTS IN MY COCKPIT THOUGHT WE HEARD OUR CALL SIGN. WHEN WE WERE DSNDING THROUGH FL284; ZID ASKED US WHAT OUR ALT WAS. WE RESPONDED AND WERE TOLD THAT THE CLRNC TO DSND WAS NOT FOR OUR CALL SIGN. ZID SUBSEQUENTLY CLRED US TO CONTINUE DOWN TO FL270. ACR X FLT ABCD WAS NOT UP ON THE FREQ; AND NEVER DID RESPOND TO FURTHER REPEATED CALLS FROM ZID. 1) BOTH OF US IN THE COCKPIT OF 'ACR X AECF' MISTOOK THE CALL DIRECTED TO ACR X ABCD. THE CALL SIGNS ARE SIMILAR IN THAT THEY BEGIN WITH THE SAME NUMBERS AND HAVE THE WORD 'X' IN THE MIDDLE. IF NOT ENUNCIATED CLRLY; AND IF RADIO RECEPTION IS SCRATCHY; THEY MAY BE MISTAKEN. 2) ACR X ABCD WAS NOT UP ON THE FREQ; AS ZID WAS EXPECTING. ACR X ABCD COULD HAVE ASSISTED IN THE ERROR IF THEY HAD BEEN ON FREQ. 3) ZID DID NOT CATCH OUR READBACK FOR THE WRONG FLT NUMBER. WE DEVIATED 600 FT FROM OUR ASSIGNED ALT BEFORE THE ERROR WAS DETECTED BY ZID. NO TFC CONFLICTS WERE APPARENT AND NO TCASII WARNINGS WERE GIVEN. WE MADE A MISTAKE AND FORTUNATELY DID NOT COME CLOSE TO ANOTHER ACFT.