BE1900 FLC HAS NMAC DURING DSCNT INTO MIA.

Narrative

WE WERE ON OUR DSCNT; TRACKING THE AIRWAY BTWN VORS. ZMA INSTRUCTED US TO CROSS DEKAC INTXN AT 4000 FT. PRIOR TO DEKAC; WE WERE LEVEL AT 4000 FT; WE WERE THEN HANDED OFF TO APCH. I HAD THE TCASII ON THE 10 MI RING. I NOTICED A TARGET AT 500 FT ABOVE US; APPROX 6 MI OR SO. I WAS NOT SURE OF THE ACFT'S DIRECTION OF FLT. WE WERE WBOUND. TO AVOID A COLLISION WITH THE ACFT; WE BANKED 30 DEGS AND CHANGED HEADING. I ADVISED MIA APCH THAT I HAD AN ACFT ON OUR TCASII AND WAS DEVIATING TO AVOID A COLLISION. AT THAT POINT; I COULD SEE AN ACFT EMERGE FROM A CLB OFF OUR PORT WING. THE ACFT WAS WHAT I APPROXIMATED TO BE 200 FT AWAY; AND WOULD HAVE; I AM SURE; COLLIDED WITH US; IF NOT FOR ACTIONS. I COULD SEE THIS ACFT WAS A WHITE DH6 TWIN OTTER WITH RED AND BLUE TRIM. I ASKED ATC WHY WE WERE NOT ADVISED OF THE ACFT. HE TOLD ME THAT HE WAS ON THE LAND LINE PHONE; AND DID NOT SEE THE OTHER ACFT UNTIL I REQUESTED DEV. I LATER ASKED HIM FOR A PHONE NUMBER TO SPEAK WITH HIS SUPVR. AFTER LNDG; I CALLED THE TRACON AND SPOKE WITH THE OPS SUVR. HE ALSO TOLD ME THAT THE CTLR WAS DISTRACTED BY HIS PHONE AND OTHER ACFT. HE ALSO SAID THAT THE UNKNOWN ACFT WAS NOT TALKING TO ANYONE AND WAS VFR; BUT WAS NOT FLYING A VFR ALT.