ACR PLT TAKES CLRNC MEANT FOR ANOTHER ACFT AND DSNDED TO OCCUPIED ALT RESULTING IN LTSS.

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ON OUR DSCNT INTO TUCSON; APCH CTL (125.10) CLRED US TO 9000 FT ON A HDG OF 180 DEGS. THE APCH CTLR APPEARED TO BE BUSY WITH MIL TFC ON UHF. ON OUR DSCNT TO 9000 FT THE CTLR CHANGED OUR ALT TO 13000 FT. WHEN WE WERE AT 13300 FT AND DSNDING; WE RESPONDED 'ACR X CLRED TO 13000 FT.' THE CTLR THEN SAID 'GO AHEAD AND MAINTAIN 12000 FT. WE RESPONDED 'ROGER DOWN TO 12000 ACR X.' WE HEARD NO RESPONSE TO THAT SO WE ASSUMED THE READBACK WAS CORRECT. DSNDING THROUGH 12300 FT OUR TCASII CALLED TFC AT 11 O'CLOCK BEYOND 6 MI. NO ALT WAS PRESENTED ON DISPLAY DUE TO THE TFC'S DISTANCE FROM US. WE THEN NOTICED THE TFC'S ALT WAS 300 FT BELOW US. I THEN STARTED LEVELING AT 12300 FT UNTIL WE COULD IDENT TFC. JUST ABOUT THE SAME TIME APCH CALLED TFC FOR US AT 10 O'CLOCK AND I BELIEVE 4 MI AT 12000 FT. THE CAPT RESPONDED 'IF HE'S AT 12000 FT WHY ARE WE CLRED TO 12000 FT.' THE CTLR RESPONDED; 'ACR X YOU WERE CLRED TO 13000 FT.' I THEN PITCHED UP AND ADDED PWR TO RETURN TO 'ASSIGNED' ALT. SHORTLY AFTER ADDING PWR AND BEGINNING MY CLB. TCASII GAVE AN RA 'MONITOR VERT SPD.' IT SHOWED A RED ARC ON IVSI IN ALL AREAS EXCEPT 2000 FPM DSCNT. SEEING THIS I RESPONDED BY REDUCING PWR AND INITIATING DSCNT. JUST THEN THE TCASII GAVE AN RA 'DSND; DSND.' I RESPONDED BY LOWERING PITCH TO INITIATE DSCNT. JUST THEN THE CONFLICTING TFC POPPED OUT OF A CLOUD INTO VIEW. HE WAS AT 12 O'CLOCK SLIGHTLY BELOW US IN A L BANK MOVING L TO R. ESTIMATED DISTANCE WAS 1/2 MI. WE THEN CALLED TO APCH 'ACR X HAS TFC IN SIGHT.' APCH THEN SAID MAINTAIN 8000 FT. WE MAINTAINED VISUAL SEPARATION FROM THE FGT AS HE PASSED L TO R IN FRONT OF US. LOOKING BACK INTO THE OCCURRENCE; I FEEL THAT THE PROB OCCURRED WHEN WE WERE ISSUED CLRNC TO 12000 FT. THE CLRNC WAS IMMEDIATELY AFTER WE ACKNOWLEDGED OUR CLRNC TO 13000 FT. WE THOUGHT THE CTLR AMENDED OUR ALT TO 12000 FT BECAUSE WE WERE APCHING 13000 FT AT SUCH A HIGH RATE OF DSCNT. I THINK THE PHRASE 'GO AHEAD AND MAINTAIN 12000 FT' WAS NOT FOR US; BUT FOR THE FGT THE CTLR WAS WORKING ON UHF.