WHEN INTERRUPTED BY INTERPHONE CALL FROM A CABIN ATTENDANT; INADVERTENTLY PLACED IN THE 'EMER MODE;' THE FLT CREW OF A DC9 IN A HOLDING PATTERN NEAR ATL FAILS TO MONITOR AIRSPD AND EXPERIENCE A STALL AND LOSS OF SEPARATION WITH ACFT BENEATH THEM.

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WHILE FLYING THE MACEY ARR INTO ATLANTA; GA; WE WERE ISSUED HOLDING INSTRUCTIONS. HOLDING WOULD BE AT THE MACEY INTXN. THE AIRPLANE WAS SLOWED PRIOR TO ARR AT THE FIX. AS WE ENTERED THE HOLDING PATTERN; I WAS ABOUT TO RPT TO ATC OUR ENTRY INTO HOLDING. IT WAS AT THIS POINT THAT WE RECEIVED A CALL FROM THE FLT ATTENDANT. ALTHOUGH THIS CALL TURNED OUT TO BE ROUTINE; IT CAME ACROSS AS AN EMER CALL FROM THE CABIN. THIS; NATURALLY; GOT THE ATTN OF BOTH CREW MEMBERS (MYSELF AND THE CAPT). AT THIS POINT; THE CAPT CALLED MY ATTN TO A VIBRATION ON THE AIRPLANE. AFTER A BRIEF MOMENT OF INVESTIGATION; WE DISCOVERED THAT THE SOURCE OF THE VIBRATION WAS A LOSS OF AIRSPD; WHICH WHEN COMBINED WITH THE BANK ANGLE OF THE ACFT; RESULTED IN A STALL OF THE AIRPLANE AND A SUBSEQUENT LOSS OF ALT (APPROX 500 FT) DURING THE RECOVERY PROCESS. RECOVERY; ALTHOUGH INITIATED IMMEDIATELY; TOOK MORE THAN 1 MIN. WE TRIED TO GET A LOWER ALT FROM ATC BUT THEY WERE UNABLE TO ACCOMMODATE THE REQUEST DUE TO OTHER TFC HOLDING BELOW US. ONCE RECOVERY WAS COMPLETE WE RETURNED IMMEDIATELY TO OUR ASSIGNED ALT. THE DISTR; ALTHOUGH MOMENTARY; WAS TIMED SO AS TO DIVERT ATTN FROM THE TASK AT HAND. IN HINDSIGHT IT MAY HAVE BEEN PRUDENT TO EMPHASIZE THE ROLE OF THE CAPT TO FLY THE AIRPLANE; WHILE I ANSWERED A CALL THAT WE THOUGHT WAS OF AN EMER NATURE; PRIOR TO ANSWERING THE CALL! I'M NOT POSITIVE THIS WOULD HAVE PREVENTED OUR SIT BUT IT MAY HAVE BEEN A GOOD REMINDER OF THE PRIORITY OF OUR TASKS. SUPPLEMENTAL INFO FROM ACN 622079: WE WERE BUSY AS WE ENTERED THE HOLDING PATTERN; SO I DECIDED TO USE A 'BALLPARK' HOLDING SPD VERSUS LOOKING ONE UP. THE BALLPARK SPD -- VREF +20 WORKS IN MOST SITS; BUT NOT IN THIS ONE. LESSON LEARNED.