AN MDT CREW; DEPARTING RSW; HAVING MISSED AN AMENDED LEVELOFF CLRNC; WERE QUERIED BY ATC AS TO THEIR INTENTIONS.

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CLBING OUT OF RSW THROUGH 10000 FT WITH A CLRNC TO CLB TO 16000 FT ON A HDG OF 310 DEGS; WE RECEIVED A CALL FROM FT MYERS APCH. THE FIRST FEW WORDS WERE UNREADABLE; FOLLOWED BY 'ZMA IS BUSY.' WE DID NOT ACKNOWLEDGE THE CALL WITH A 'ROGER' OR ANYTHING ELSE BECAUSE IT DID NOT SOUND LIKE A CLRNC AT ALL. A FEW SECONDS LATER; CLBING 3200 FPM; WE RECEIVED A CALL FROM ATC SAYING 'WHAT ARE YOU DOING?' I SAID 'CLBING THROUGH 12000 FT FOR 16000 FT HDG 310 DEGS.' ATC SAID THAT WE WERE ONLY CLRED TO 10000 FT AND TO DSND BACK TO 10000 FT. WE IMMEDIATELY DSNDED TO 10000 FT. THERE WAS NO CONFLICT WITH ANY OTHER AIRPLANES. I BELIEVE WHAT CAUSED THE PROB WAS THAT THE ATC CTLR DID NOT USE STANDARD PHRASEOLOGY AND DID NOT GET A READBACK FROM US; IF INDEED ATC TOLD US STOP AT 10000 FT. AS FAR AS WE KNOW WE DID NOT RECEIVE A CLRNC TO STOP CLB AT 10000 FT. IF ATC TOLD US TO STOP CLB AT 10000 FT; ATC SHOULD NOT EXPECT US TO BE ABLE TO NOT GO OVER 10000 FT WHEN ATC TOLD US AT 10000 FT AND WE WERE CLBING AT 3200 FPM.