WHILE DSNDING IN DEN TRACON AIRSPACE; AN ACR D328 FLC EXECUTES EVASIVE TCASII MANEUVER REACTING TO TFC BELOW; CLBING AND CONVERGING. FOLLOW-UP CONVERSATION WITH TRACON SUPVR REVEALS CTLR INCORRECTLY ASSIGNED AN ALT THAT WOULD CONFLICT WITH CLBING TFC.

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WHILE APCHING DEN ARPT FOR LNDG; AN INCORRECT ALT ASSIGNMENT WAS APPARENTLY ISSUED BY ATC. WE HAD RECEIVED A HDOF FROM ZDV WHO HAD ASSIGNED 17000 FT. UPON CHK-IN; DEN APCH ASSIGNED A LOWER ALT OF 15000 FT. SOON AFTERWARD WE WERE GIVEN A HDG OF 080 DEGS FOR TFC AND A NEW ALT ASSIGNMENT OF 12000 FT. WHILE DSNDING TO 12000 FT; WE WERE GIVEN A HDG OF 150 DEGS. AT THIS TIME; I LEFT THE ATC FREQ TO CONTACT OUR COMPANY OPS ON A SECOND RADIO. SIMULTANEOUS WITH MY CALL TO OPS; WE RECEIVED A TCASII TA FOR TFC APCHING APPROX 3 MI FROM OUR L AND 400 FT BELOW. WE WERE DSNDING THROUGH 12400 FT. AT THE SAME TIME I COULD HEAR ATC ADVISING US OUR ASSIGNED ALT WAS 13000 FT AND TO CLB AND MAINTAIN 13000 FT. I SPOTTED THE TFC CAUSING THE TA AND SWITCHED BACK TO THE ATC FREQ. AGAIN; WE WERE ASSIGNED 13000 FT; AND ADVISED THAT OUR LAST ASSIGNED ALT HAD BEEN 12000 FT; TO WHICH THE CTLR REPLIED HAD NEVER ASSIGNED. WE WERE HANDED OFF TO THE NEXT CTLR AND THE BAL OF THE FLT WAS UNEVENTFUL. ON THIS PARTICULAR FLT WE HAD A PLT OCCUPYING OUR COCKPIT JUMP SEAT. ON CONFERRING WITH HIM ON THE GND; WE FOUND HE'D ALSO HEARD THE 12000 FT ASSIGNMENT. A SUBSEQUENT TELEPHONE CALL FROM DEN TRACON REVEALED THEY HAD REVIEWED THE RECORDING AND FOUND THAT THE CTLR HAD INDEED ASSIGNED 12000 FT.