AN ACR B737-200 FLC GETS DISTRACTED WHILE DSNDING AND GOES BELOW 9000 FT ASSIGNED.

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APCHING PHX ON THE KARLO 6 ARR; FO FLYING; DSNDING TO ASSIGNED ALT TO 9000 FT. FO SUGGESTED WE ASK FOR THE N RWY; ASSUMING BY OUR ALT ASSIGNMENT OF 9000 FT SO CLOSE IN THAT WE WERE BEING VECTORED FOR THE S RWY. (OUR GATE WAS ON THE N SIDE.) JUST AFTER I MADE OUR REQUEST TO APCH AND RECEIVED A RESPONSE; I LOOKED DOWN AT OUR ALT WHICH WAS PASSING THROUGH ABOUT 8800 FT. I IMMEDIATELY RESPONDED VERBALLY TO FO AS I REACHED FOR THE CTLS. FO REACTED IMMEDIATELY WITH PWR AND PULLED THE NOSE UP. AS WE BOTH WERE REACTING; APCH TOLD US TO CLB TO 9000 FT. BY THE TIME WE NOTICED THE DEV AND THEN REACTED; OUR ALT BOTTOMED OUT AROUND 8650 FT. WE ARE NORMALLY TURNED IN ON THE N SIDE OF THE ARPT FOR A R DOWNWIND TO THE S RWY WHILE ON THE KARLO 6 ARR. OUR DISCUSSION AS TO WHAT EXACTLY APCH WAS GOING TO DO WITH US; DISTRACTED US FROM LEVELING OFF AT ASSIGNED ALT. PERSONALLY; MY FIX FOR THE PROB WILL BE SIMPLE. WHEN THE ALT WARNING (1000 FT TO GO) COMES ON; I WILL NOT ALLOW MYSELF TO BE DISTRACTED UNTIL LEVEL OFF. HAVING A DEFINITE PLAN PRIOR TO THE ARPT IS A PLUS. HAD WE KNOWN FOR SURE WHAT APCH CTL HAD IN MIND; THIS DISCUSSION WOULD PROBABLY NOT HAVE EVEN TAKEN PLACE AND THE ALT EXCURSION NOT HAPPENED. MOST OF THE TIME; ACCOMPLISHING MULTIPLE TASKS ALL AT THE SAME TIME IS A STANDARD WHILE FLYING. THROW IN A LITTLE NON NORMAL AT JUST THE WRONG MOMENT; IT'S NOT DIFFICULT TO GET DISTRACTED.