A B737-700 FLC OVERSHOOTS THEIR ASSIGNED ALT WHEN THE FO SETS THE WRONG ALT IN THE ALT ALERTER 10 MI W OF GPT; MS.

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PERFECT WX; BRAND NEW AIRPLANE; EXTREMELY SHARP FO. DSNDING INTO MSY (DIRECT TO GPT); I WAS OFF TO GET ATIS. FO (PF) SPOKE TO ATC AND PUT FL230 IN MCP. WHEN I CAME BACK FROM GETTING PART OF THE ATIS; ATC FREQ WAS EXTREMELY CONGESTED; INCLUDING A STUCK MIKE. I SAID 'FL230?' POINTING TO MCP; FO NODDED; FLT ATTENDANT ENTERED THE COCKPIT; ENGAGED US IN CONVERSATION. I LEFT THE FREQ BRIEFLY TO GET THE REST OF THE ATIS AND WHEN I RETURNED; AS WE LEVELED AT FL230; ATC QUERIED US ON OUR ALT; SAID WE WERE CLRED ONLY FL240; AND THEN CLRED US DOWN TO LOWER ALT. I FEEL THE EXTREME CONGESTION AND STUCK MIKE ON THE FREQ CONTRIBUTED SIGNIFICANTLY TO CAUSE THE CONFUSION WHICH RESULTED IN THIS EVENT. SUPPLEMENTAL INFO FROM ACN 489379: THE CAPT WAS OFF GETTING THE ATIS. ZHU GAVE ME A DSCNT CLRNC TO FL240. I THOUGHT I READ BACK FL240 AND PUT FL240 IN THE WINDOW. I'M NOT SURE HOW; BUT FL230 GOT PUT INTO THE WINDOW. 1 OF 2 THINGS HAPPENED. I PUT FL230 IN THE WINDOW BUT READ BACK FL240; OR MY CAPT ANSWERED FL230 FOR ANOTHER ACFT AND PUT FL230 INTO THE WINDOW.