FLC WAS ISSUED A VERY LOW ALT TO DEST. ENRTE THE FLT WAS BRIEFLY CLRED TO FL230 THEN INSTRUCTED TO DSND BACK TO 17000 FT. WHEN THE CAPT QUESTIONED THE CHANGE; ANOTHER VOICE ADVISED THAT THE CREW HAD TAKEN THE WRONG CLRNC.

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WE WERE ON A SCHEDULED FLT FROM FLL TO MCO. WITH TSTMS ALONG OUR RTE OF FLT; WE WERE ASSIGNED A NORTHERLY HEADING BY ATC; AND AN ALT OF 17000 FT. DUE TO TFC; ETC; WE WERE ADVISED THAT 17000 FT WOULD PROBABLY BE OUR FINAL ALT; WHICH WAS SIGNIFICANTLY LOWER THAN NORMAL. MINS LATER; ZMA THEN CLRED US TO FL230. I ACKNOWLEDGED THE CLRNC (WITH FULL CALL SIGN AND READBACK). THE CAPT ALSO HEARD THE SAME CLRNC AND PROMPTLY STARTED PROGRAMMING THE FMC AND VERIFIED I HAD DIALED THE ALT INTO THE ALT ALERT WINDOW. A FEW MINS LATER THE SAME CTLR THEN CLRED US BACK TO 17000 FT. I TRIED TO NEGOTIATE TO STAY AT FL230 BECAUSE IT WOULD PUT US ABOVE MOST OF THE CUMULUS BUILDUPS; BUT THE CTLR WAS INSISTENT THAT WE DSND BACK TO 17000 FT. WHEN THE CAPT QUESTIONED WHY THE CLRNC WAS CHANGED; ANOTHER VOICE APPEARED AND SAID WE HAD TAKEN THE WRONG CLRNC. BOTH THE CAPT AND MYSELF ARE ABSOLUTELY SURE THE FIRST CTLR USED OUR CALL SIGN FOR THE ALT CHANGE. AFTER FURTHER DISCUSSION; WE HYPOTHESIZED THAT THE CTLR MISTAKENLY ISSUED THE CLRNC TO US; THAT IS WHY HE REISSUED A NEW CLRNC TO 17000 FT. NEVER WERE OUR ACTIONS QUESTIONED BY ATC; AND IT WAS THE WORDING USED BY THE SUPVR (OTHER VOICE) THAT PUT SOME DOUBT IN OUR MINDS. IF THE CLRNC WAS INTENDED FOR ANOTHER ACFT; THEN I BELIEVE THE CTLR COULD HAVE WORDED HIS RESPONSE BETTER.