A DSNDING B737-300 OVERSHOOTS ITS ASSIGNED ALT WHEN THE FLC FAILS TO ASK THE CTLR TO REPEAT THE CLRED TO ALT.

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FLT WAS LAS TO PHX. WHILE ON KARLO 7 ARR INTO PHX; PHX APCH CTL ISSUED DSCNT TO 9000 FT AT PLSNT INTXN. WE WERE STILL 3-4 MI FROM PLSNT INTXN. I HEARD 9000 FT; BUT WAS NOT SURE. COPLT WAS SURE HE HEARD 8000 FT. WE DISCUSSED IT AND SINCE THE CTLR WAS BUSY; WE DECIDED THAT WE WOULD RPT OUT OF 12000 FT AND CLRLY ANNUNCIATE 'OUT OF 12000 FT FOR 8000 FT' AND THAT I WOULD ACCOMPLISH A SLOW DSCNT TO BE SURE. WHEN WE RPTED OUT OF 12000 FT FOR 8000 FT; THE CTLR ASKED FOR US TO REPEAT IT. WE AGAIN SAID OUT OF 12000 FT FOR 8000 FT. WE DSNDED TOWARD 8000 FT. AT ABOUT 8300 FT THE CTLR TOLD US TO CLB TO 9000 FT AND SAID THAT HAD BEEN THE ALT TO WHICH WE WERE CLRED. THERE DID NOT SEEM TO BE A TFC CONFLICT; TCASII OR OTHERWISE; AND THE CTLR DID NOT SEEM OVERLY EXCITED. I RAPIDLY CLBED BACK TO 9000 FT. NOTHING WAS EVER SAID; FROM THAT POINT ON; ABOUT A POSSIBLE ALT VIOLATION. SUPPLEMENTAL INFO FROM ACN 432014: THE CAPT QUESTIONED ME AS TO THE ALT. HE THOUGHT IT MIGHT BE 9000 FT. HE WASN'T SURE. I CONFIRMED THE 8000 FT ALT BY CALLING APCH CTL AND RPTING; 'OUT OF 12000 FT FOR 8000 FT.' APCH CTL ASKED US TO REPEAT; WHICH WE DID. AT 8300 FT APCH CTL CALLED US WITH AN ALT; 'CALL SIGN; 9000 FT.' I CALLED APCH CTL AND SAID; 'WE CONFIRMED 8000 FT OUT OF 12000 FT.' HIS RESPONSE WAS 'MAINTAIN 9000 FT.'