AN ACR MD88 FLC DSNDED BELOW THEIR ASSIGNED ALT.

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IN CRUISE ON A FLT FROM IND TO ATL; FLT ATTENDANT CALLED AND THEN WAS LET INTO COCKPIT TO OFFER DRINKS AND DISCUSS FLT DETAILS. SHE ENTERED AND WAS TALKING TO THE CAPT WHEN ATC (ZTL) ISSUED A DSCNT. I ANSWERED BY SAYING WE WERE DSNDING TO FL230. PASSING FL317 CTLR TOLD US TO CLB TO FL330. WE STOPPED DSCNT AT FL315 AND COMMENCED THE CLB. WE RECEIVED A TA FROM TCASII AND THE CTLR TOLD US TO QUICKLY CLB TO FL330. WE COMPLIED. RPTEDLY THE CTLR HAD ISSUED THE DSCNT FOR FL330. CONTRIBUTING TO THIS MISUNDERSTANDING WERE THE FOLLOWING FACTORS: 1) MY MISHEARING OF THE CLRNC DUE TO THE CONVERSATION IN THE COCKPIT (EVEN THOUGH IT WAS PERTINENT TO THE FLT). 2) THE CAPT NOT HEARING THE CLRNC. 3) THE CTLR NOT CATCHING THE ERRONEOUS READBACK. SUPPLEMENTAL INFO FROM ACN 387850: I WAS JUST FINISHING UP LISTENING TO AN UPDATED ATIS WHEN THE CABIN ATTENDANT GAVE THE COCKPIT ENTRY SIGNAL AT THE SAME TIME ATC GAVE US A CLRNC TO DSND AND MAINTAIN FL230. THE PF (FO) SET THE ALT REMINDER AND REPLIED 'DSNDING TO FL230.' PASSING THROUGH FL317 ATC SAID 'CLB AND MAINTAIN FL330.' AFTER LEVEL; THE CTLR ASKED WHERE WE WERE DSNDING TO. I REPLIED 'WE WERE DSNDING; CLRED AND READ BACK FL230.' HE REPLIED; 'NO; YOU WERE CLRED TO FL330.' WHEN ATC LISTENED TO THE TAPES LATER; THEY SAID THE CTLR CLRLY SAID FL330; BUT THE FO CLRLY READ BACK FL230.