ALTDEV ALT OVERSHOT AND ALT XING RESTRICTION NOT MET.

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WHILE IN CRUISE AND PRIOR TO DSCNT FROM FL410; CHIEF PURSER ASKED SO TO CHK ON CLOSE CONNECTIONS FOR PAX UPON OUR ARR. SO BEGAN USING ACARS RADIO TO DO SO. A SHORT TIME LATER; STILL IN CRUISE; THE FO ASKED TO BE EXCUSED FOR 'PHYSIOLOGICAL REASONS.' SHORTLY AFTER THAT ATC CLRED US TO CROSS 85 MI S OF FAK VOR AT FL230. I READ IT BACK; PUT IT IN THE CLRED ALT WINDOW (23000 FT) AND BEGAN DSCNT TO COMPLY WITH ISSUED CLRNC. DURING DSCNT FO RETURNED TO COCKPIT AND I ADVISED HIM OF THE CLRNC. HE THEN BEGAN FLYING AGAIN AND DSNDED TO FL230. SHORTLY AFTER LEVEL OFF AT FL230 THE ATC CTLR REQUESTED OUR ALT. I RESPONDED 'LEVEL AT FL230.' HE CAME BACK WITH A CLB CLRNC TO FL240; WHICH WE DID. HE SAID HE HAD CLRED US TO FL270!? I THOUGHT I HEARD FL230; AND DUE TO MINOR DISTRS (I.E.; SO ON COMPANY RADIO) AND PHYSIOLOGICAL NEED (OF FO TO THE LAVATORY) I WAS THE ONLY ONE WHO HEARD THE CLRNC! SUPPLEMENTAL INFORMATION FROM ACN 244957: IT WAS A EASY AND HONEST MISTAKE; BUT WE ARE STILL UNSURE OF WHO WAS AT FAULT -- ATC OR CAPT; WHO DIDN'T HEAR PROPER READBACK. IN FUTURE I BELIEVE A MINIMUM OF 2 CREW MEMBERS SHOULD HEAR AND CONFIRM ALL ATC INSTRUCTIONS. IF A 2-MAN CREW; MAYBE INSTRUCTIONS SHOULD BE CONFIRMED WITH ATC UPON RETURN OF MISSING CREW MEMBER.