A B737 FLC OVERSHOT THEIR REASSIGNED ALT WHEN THE PIC PF WAS DISTR BY CONVERSATION WITH A FLT ATTENDANT. ATC CORRECTED ALT OVERSHOT.

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MYSELF (CAPT) FLYING; FO ON VHF. DURING ENRTE CLB; ZTL HAD CLRED US TO FL330. FO READ BACK TO ATC FL330 AS I SET IT IN THE ALT SELECT WINDOW. BOTH OF US POINTED AND SAID ALOUD '330.' ACFT WAS CLBING IN VNAV. FLT ATTENDANT CAME INTO COCKPIT AND WAS TALKING TO US. APPARENTLY ATC REVISED CLB TO FL310 DUE TO TFC. I DID NOT HEAR THIS REVISION AND THEREFORE DIDN'T RESET ALT WINDOW. FO APPARENTLY HEARD IT AND READ BACK '310' (IT IS OUR POLICY TO BOTH POINT OUT NEW ALT AND FOR PF TO RESET IT AND THEN BOTH OF US POINT AND SAY NEW ALT). I DIDN'T HEAR IT AND THEREFORE DIDN'T CHANGE IT FROM FL330. AS WE CLBED THROUGH FL315; ATC CONTACTED US AND TOLD US TO RETURN TO FL310. WE COMPLIED ASAP AND REACHED FL316 BEFORE DSNDING. WE CONTINUED ON FLT ULTIMATELY AT FL350. LATER ON IN THE FLT; WE WERE ASKED TO CALL ZTL. I DID SO ON ARR AT MCI. THEY INFORMED ME A RPT HAD BEEN FILED AND ASKED ME FOR MY NAME; ADDRESS AND PHONE NUMBER. TIME NOW WAS 1 HR AFTER ARR IN MCI. DISTR WAS FLT ATTENDANT TALKING TO US. INACTION WAS FO READING BACK FL310 CLRNC AND NOT CALLING IT TO MY ATTN SO ALT WINDOW COULD BE CHANGED OR NOT CHANGING IT HIMSELF. INACTION WAS MY NOT HEARING THE REVISED ALT. WHY WERE WE CLRED TO FL330 IF SEPARATION WAS INADEQUATE?