LOSS OF SEPARATION OCCURS DURING AN ALT EXCURSION BTWN A DSNDING MD80 AND AN IN CRUISE CARJ AT FL190 100 NM NE OF VKZ; FL.

Narrative

CAPT WAS PERFORMING RADIO CALLS; CHKLISTS. FO WAS FLYING. ZMA GAVE PLT'S DISCRETION DSCNT FROM FL310 TO FL240. BEGAN DSCNT AT 2500 FPM. ABOUT FL280 WE WERE CLRED TO FL200. BOTH PLTS RECALL SETTING AND ARMING FL240 AND THEN FL200. WHILE STILL DSNDING TO FL200; CAPT MADE INBOUND CALL TO OPS AND WAS OFF PRIMARY ATC RADIO FOR BOUT 1 MIN. AFTER THE INBOUND CALL; CAPT ACKNOWLEDGED 'BACK ON' AND NOTICED THE ALT WAS SET AT 17000 FT. CAPT STATED TO FO; 'UNDERSTAND CLRED TO 17000 FT.' CAPT DOES NOT RECALL FO CONFIRMING 17000 FT. DSCNT CHKS BEGAN AT FL220. AT APPROX FL180; WE GOT A TCASII TFC ALERT WITH TFC ABOUT 2 O'CLOCK POS AND 5-6 MI. CAPT SAW THE ACFT AND DETERMINED NO CONFLICT. SHORTLY AFTERWARDS; WE GOT AN RA TO DSND; WHICH WE ALREADY WERE. ZMA ASKED US IF WE HAD THE TFC AND CAPT ACKNOWLEDGED THAT WE HAD. THE CTLR MENTIONED SOMETHING ABOUT FL200 AND I STATED WE WERE CLRED TO 17000 FT. HE STATED WE WERE CLRED TO FL200 BUT SINCE WE WERE NOW BELOW THE TFC; CONTINUE DSCNT TO 17000 FT. I HONESTLY DO NOT RECALL SETTING 17000 FT; NOR DO I KNOW HOW IT GOT THERE. I THOUGHT THE FO HAD SET IT AND WE WERE CLRED TO 17000 FT WHILE I WAS OFF THE RADIO MAKING THE INBOUND RADIO CALL. I SHOULD HAVE AGGRESSIVELY DEMANDED A VERBAL CONFIRMATION FROM THE FO OR ATC ON THE CHANGED ALT SETTING.