ALT EXCURSION OCCURS WHEN AN ACR FLT CREW INTERPRETS A TA FOR TFC AT A LOWER ALT AS AN ALT ASSIGNMENT; 25 MI W OF ATL; GA.

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WHILE ON THE LAGRANGE ARR INTO ATL; ZTL GAVE US SEVERAL 1000 FT STEP-DOWN ALT ASSIGNMENTS. THE LAST ALT ASSIGNMENT WAS 12000 FT AND WE WERE TOLD ABOUT TFC AT 11000 FT (A BEECH KING AIR). WE RPTED THE TFC IN SIGHT (IT WAS PASSING BEHIND US AND DIVERGING) AND WE WERE HANDED OFF TO ATLANTA APCH. ATLANTA APCH ASSIGNED US 11000 FT (WE THOUGHT) AND I READ BACK THE DSCNT CLRNC AS; 'DOWN TO ONE ONE; ELEVEN THOUSAND; ACR X.' I THEN SET 11000 FT IN THE ALT ALERTER AND CONFIRMED THE ALT ASSIGNMENT WITH MY FO (HE WAS PF AND I WAS PNF). I THEN SWITCHED TO COM #2 TO CALL OUR COMPANY AND GET A GATE ASSIGNMENT. WHILE I WAS TALKING WITH OUR COMPANY; THE APCH CTLR QUERIED THE FO ON OUR ALT; WHICH WAS 11500 FT. THE CTLR STATED THAT HE WAS ONLY POINTING OUT TFC AT 11000 FT AND NOT GIVING US A DSCNT CLRNC; HE THEN CLRED US TO 8000 FT. THERE WAS NO TFC CONFLICT. THE REMAINDER OF THE FLT WAS UNEVENTFUL. THIS MISUNDERSTANDING COULD HAVE BEEN EASILY AVOIDED HAD THE CTLR LISTENED TO AND CORRECTED MY INCORRECT READBACK OF WHAT WAS OBVIOUSLY AN ALT CLRNC AND NOT AN ACKNOWLEDGEMENT OF TFC. IT'S UNFORTUNATE THAT THE FAA FEELS THEY ARE NOT RESPONSIBLE FOR LISTENING TO AND CORRECTING INCORRECT READBACKS FROM PLTS; IT REMOVES A VITAL SAFETY LINK. THE CTLR WAS NOT UNUSUALLY BUSY; NOR WERE WE. I'M NOT SURE WHAT ELSE I COULD HAVE DONE TO PREVENT THIS. BOTH OF US WERE CERTAIN WE WERE GIVEN A DSCNT CLRNC AND I READ IT BACK CLRLY ('ONE; ONE; ELEVEN THOUSAND'). WE ALSO AUDIBLY CONFIRMED THE ALT ASSIGNMENT BY STATING 'ELEVEN THOUSAND' TO EACH OTHER AFTER THE ALT ALERTER WAS SET. FORTUNATELY THERE WERE NO TFC CONFLICTS. FORTUNATELY MOST CTLRS LISTEN TO READBACKS AND CORRECT INCORRECT CLRNCS.