FOLLOWING A LATE RWY CHANGE APPROACHING AUS; A B737-300 CREW LOST SEPARATION WITH AN RJ BECAUSE OF MISSED RADIO CALLS WHILE DOING FMS WORK.

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WHILE BEING VECTORED FOR A VISUAL APCH TO RWY 17L AT BERGSTROM INTL ARPT; AUSTIN; TX; APCH CTL SWITCHED US RWY 17R AND TURNED US TO HDG 350 DEGS. WHILE THE FO (PLT MONITORING) AND I (PF) WERE DISCUSSING THE CHANGE AND SETTING UP FOR RWY 17R; WE MISSED APCH CTL CALLING US WITH HDG 010 DEGS AND DSCNT TO 3000 FT. THE SECOND TIME THEY CALLED US; I MISSED THE HDG AND ALT; BUT UNDERSTOOD THAT THEY WANTED US TO TURN. AT THE SAME TIME; I SAW A TARGET ON OUR TCAS TURN YELLOW. IT WAS IN OUR 12 O'CLOCK POS AT OUR ALT. I ASSUMED THAT WAS TFC WE WERE TURNING FOR AND THAT APCH HAD GIVEN US A BASE TURN. I STARTED A TURN TO THE W TO AVOID THE TFC AND TURN ONTO BASE WHILE THE FO ATTEMPTED TO CALL APCH TO CLARIFY OUR CLRNC. BEFORE THE FO COULD CALL; APCH CALLED THE REGIONAL JET THAT WAS AT OUR 12 O'CLOCK POS AND HAD THEM TURN L. BY THE TIME WE FIGURED OUT THAT WE WERE SUPPOSED TO HAVE TURNED R TO 010 DEGS AND CONTINUED ON DOWNWIND INSTEAD OF L FOR A BASE TURN; IT WAS SAFER TO CONTINUE TO TURN W TO AVOID THE REGIONAL JET. WE MAINTAINED VISUAL CONTACT WITH THE RJ AND REMAINED WELL CLR AND NEVER RECEIVED A TCAS TA OR RA. WE TOLD ATC WHAT WE HAD DONE AND ASKED IF THERE WAS A PROB. THEY REPLIED THAT THEY WERE NOT SURE IF THERE WAS A SEPARATION PROB BTWN US AND THE RJ; BUT GAVE US A PHONE NUMBER TO CALL WHEN WE LANDED. I CALLED AND AFTER THEY REVIEWED THE TAPES; THEY SAID THAT THERE WAS A LOSS OF SEPARATION BTWN US AND THE RJ AND THAT THEY WERE GOING TO FILE A RPT. DON'T GET SO WRAPPED UP IN SETTING UP NAVAIDS FOR A DIFFERENT RWY THAT YOU MISS RADIO CALLS. DON'T ASSUME YOU KNOW WHAT ATC WAS CLRING YOU TO DO. IF YOU DON'T HEAR THE FULL CLRNC; CLARIFY IT.