ALTDEV DURING DSCNT.

Narrative

AFTER INSTRUCTED TO HOLD AT MACEY ON OUR ARR INTO ATL WE WERE TOLD TO DSND TO FL240. THE COPLT READ BACK FL240 AND WE STARTED OUR DSCNT. WE WERE BEING WHAT WE THOUGHT WAS VERY CONSCIOUS OF OUR ALT BECAUSE; JUST A FEW MINS BEFORE ON A PREVIOUS FLT; WE HAD DISCOVERED OUR ALT ARM FUNCTION OF THE AUTOPLT WAS NOT WORKING. I; AS FE HAD MISSED THE DSCNT AND HOLD INSTRUCTION BECAUSE I WAS BUSY GETTING THE ATIS; GETTING A GATE ASSIGNMENT AND RPTING TO OUR MAINT COORDINATOR OUR ALT ARM MALFUNCTION. WHEN I CAME BACK IN THE LOOP; ATC ASKED US WHAT ALT WE WERE AT. WE RESPONDED FL240. HE SAID HE THOUGHT HE HAD ASSIGNED US FL250; BUT NO PROB; REMAIN AT FL240. WE TOLD HIM WE UNDERSTOOD AND READ BACK FL240 AND THANKED HIM. ALTHOUGH WE STILL DON'T KNOW WHO EXACTLY WAS AT FAULT; WE DID EVERYTHING PRESENTLY IN PLACE TO PREVENT SUCH AN ERROR. WE HAD 2 PLTS MONITORING THE RADIO; CONFIRMED THE CORRECT ALT WAS SET IN THE ALT WINDOW; READ BACK THE ASSIGNED ALT AND LEVELED AT THE PROPER ALT. IT'S THE SAME OLD COM PROB. WE CAN ONLY WORK TO PREVENT IT FROM HAPPENING BY HAVING BOTH ENDS OF THE RADIO BEING AS CONSCIENTIOUS ABOUT A CLRNCS AND READBACKS AS POSSIBLE UNTIL DATALINK CLRNCS COME ALONG.