FLC OF A B757 OVERSHOT ASSIGNED STAR FIX DURING DSCNT DUE TO DISTRS AND CLRNC CONFUSION.

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ON DSCNT INTO PHX ON FOSSL 4 ARR; ZAB CLRED US TO CROSS TONTO INTXN AT 12000 FT; 280 KTS. IN THE MEANWHILE; ZAB WAS VERY BUSY WITH TFC. AFTER A TIME; ZAB ASKED US IF WE COULD START DOWN. THE COPLT WAS FLYING THE LEG AND WE BOTH REALIZED THAT ALTHOUGH WE WERE IN VNAV; THE ALT FOR TONTO HAD NOT BEEN DIALED IN THE WINDOW ON THE MCP AND WE WERE HIGH ON THE VNAV PATH. WE WERE SCRAMBLING TO START DOWN WHEN ZAB ASKED US IF WE WOULD CONTACT ONE OF OUR SISTER FLTS TO GET THEM ON THE CORRECT FREQ. BTWN THE COPLT AND MYSELF SWITCHING TO EITHER GUARD FREQ; PA AND ACARS TO ATTEMPT CONTACT WITH THE SISTER FLT AND ACCOMPLISHING OTHER DUTIES AT VARIOUS TIMES; ONE OF US WAS OFF THE ATC FREQ. AFTER NOT RAISING OUR SISTER FLT; I CAME BACK ON ZAB AND HEARD OUR SISTER FLT CHKING IN WITH ZAB. ZAB WAS STILL VERY BUSY AND SWITCHED US TO P50 APCH JUST BEFORE TONTO. JUST PRIOR TO TONTO; I NOTICED THE ALT WINDOW IN THE MCP HAD BEEN DIALED TO 11000 FT AND THE ACFT WAS SLOWLY DSNDING TO 11000 FT. AT ABOUT 11600 FT APCH ASKED US IF WE WERE ALREADY DSNDING TO 11000 FT. I RESPONDED WE WERE AS PREVIOUSLY CLRED -- EVEN THOUGH I HAD NOT HEARD THAT ATC CLRNC IN THE CONFUSION OF ATC CHATTER AND TRYING TO RAISE OUR SISTER FLT ON ANOTHER FREQ. I WOULD NOT EVEN QUESTION THE ALT CLRNC EXCEPT FOR APCH'S TONE IN QUESTIONING US UPON INITIAL CONTACT. HE SAID NOTHING IN RETURN AND WE PROCEEDED TO DSND TO 11000 FT. REST OF ARR NORMAL.