ACR X ON WRONG FREQ WITH SIMILAR CALL SIGN. ACFT WAS MISTAKENLY ISSUED DSCNT. PENETRATED ADJOINING AIRSPACE AT WRONG ALT.

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ACR X ENCOUNTERED CONTINUOUS LIGHT CHOP WHILE CRUISING AT FL310 AND ASKED ZTL FOR A RIDE RPT. ATL HANDED US OFF TO MEMPHIS; EXPLAINING THEY WOULD HAVE BETTER INFO REGARDING THE FLT CONDITIONS. ZME RPTED A SMOOTH RIDE AT ALL ALTS. WE ASKED FOR LOWER; RPTING THE CONTINUOUS LIGHT CHOP AT FL310. WE WERE CLRED TO FL280. AFTER CRUISING AT FL280 FOR A WHILE; THE CAPT AND I BEGAN DISCUSSING THE NEED TO BEGIN OUR DSCNT FOR OUR ARR AT JACKSON; MS; WHEN WE HEARD THE CTLR ASK ANOTHER COMPANY ACFT FLT (Y123 VERSUS X123) WHAT ALT THEY WERE AT. THIS WAS THE FIRST TIME WE'D HEARD THIS CALL SIGN ON THE FREQ; AND; DUE TO THE SIMILARITY OF THE CALL SIGNS AND NATURE OF THE QUESTION; WE IMMEDIATELY WONDERED IF THE 2 CALL SIGNS AND BEEN CONFUSED. WHEN WE ASKED THE CTLR FOR A LOWER ALT; HE TOLD US WE WERE ON THE WRONG FREQ AND HANDED US OFF TO ANOTHER MEM CTLR. WHEN WE CHKED IN; THE NEW CTLR IMMEDIATELY ASKED US WHO HAD CLRED US TO FL280. WHEN WE REPLIED IT WAS THE PREVIOUS CTLR; HE ASKED FOR THE FREQ. THE REST OF THE FLT PROCEEDED WITHOUT INCIDENT; BUT WE WERE FAIRLY CERTAIN THE 2 CALL SIGNS HAD BEEN CONFUSED AND WE WEREN'T SUPPOSED TO HAVE BEEN AT FL280. ON THE GND; THE CAPT CONFIRMED THIS VIA TELEPHONE WITH ZME. THE SUPVR EXPLAINED THE CTLR WAS NEW AND HAD CONFUSED OUR CALL SIGNS. FORTUNATELY; THERE NEVER WAS A TFC CONFLICT.