CPR BE40 FLC REQUIRED TO INITIATE EVASIVE TCASII MANEUVER WHEN AN ARTCC CTLR INADVERTENTLY ASSIGNS A LOWER ALT TO THE BE40 IN CONFLICT WITH XING ACR B727 TFC. LATER COM WITH ARTCC SUPVR IDENTS CTLR ERROR.

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WE WERE DSNDING TO FL350 WHILE FLYING THE LLAKE 2 ARR INTO PBI VIA THE LAKELAND TRANSITION. ATC THEN ASSIGNED US TO 'DSND AND MAINTAIN FL230; TFC 12 O'CLOCK; FL310; A B727.' THE CAPT (PNF) READ BACK 'FL230; LOOKING FOR TFC.' AS WE DSNDED THROUGH FL325; THE B727 BECAME VISIBLE ON THE 20 NM RING OF OUR TCASII AT FL310 FT AND 12 O'CLOCK. I (FO AND PF) WAS FLYING THE ACFT AND LEVELED OFF AT FL320 WHILE THE CAPT ASKED CTR TO VERIFY OUR ALT. HE THEN TOLD THE CTR THAT 'WE HAVE A TCASII ADVISORY ON THAT TFC AT FL310.' ATC REPLIED '[CALL SIGN] (US) CLB AND MAINTAIN FL330.' I PITCHED THE ACFT UP AS THE TCASII RA WENT OFF ADVISING 'MONITOR VERT SPD.' AT THIS TIME THE PLT OF THE B727 RPTED 'DSNDING OUT OF FL310 ON A TCASII RA.' THE CTLR REPLIED 'ROGER.' I THEN WATCHED THE B727 PASS UNDER OUR R WING 1000 FT BELOW US. THE B727 PLT NEXT ASKED CTR 'WHAT CAUSED THAT TO HAPPEN.' ATC ADVISED THAT 'THE BEECHJET DSNDED BELOW HIS ASSIGNED ALT OF FL330.' WHEN THE CAPT (US) QUERIED THE CTR ON OUR ASSIGNED ALT; WE RECEIVED NO REPLY. AFTER WE CLRED THE CONFLICT; CTR ASSIGNED US FL240 AND HANDED US OFF TO THE LOW SECTOR AND SAID 'HAVE A GREAT DAY.' OUR ACFT WAS ASSIGNED FL230 INITIALLY AND WE READ BACK FL230. THE CTLR MUST HAVE THOUGHT THAT HE HAD GIVEN US FL330. WITHOUT TCASII; WE WOULDN'T HAVE KNOWN HOW CLOSE THE ACR WAS UNTIL WE HAD HIM VISUALLY -- IF WE EVEN SAW HIM VISUALLY. THIS INCIDENT REALLY SHOWS THE NEED FOR TCASII IN ALL ACFT. AFTER WE LANDED IN PBI; THE CAPT CALLED ZMA AIR TFC SUPVR AND FOUND OUT THAT THE CTR HAD A LOSS OF SEPARATION EVENT. THE SUPVR TOLD ME THAT THE CTLR HAD ASSIGNED US THE WRONG ALT OF FL230; AND THEY WERE DOING AN INVESTIGATION OF THE INCIDENT. THE ATC SUPVR APOLOGIZED FOR THE INCIDENT.