D01 CTLR COM MIX-UP RESULTS IN AN LJ31 DSNDING TO AN ALT ISSUED BY THE CTLR EXPECTING THE ACFT TO BE STILL ON THE GND AT FNL.

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THIS IS A COM ERROR THAT I THINK EVERYONE CAN LEARN FROM. WE HAD JUST DEPARTED RWY 16R AT FT COLLINS; ENRTE TO MINNEAPOLIS. WE DEPARTED VFR AND WERE CLBING THROUGH 10500 FT MSL AS MY COPLT TRIED TO PICK UP OUR CLRNC FROM DENVER. WE HAD PAX ON BOARD. THE COPLT TOLD ATC THAT WE HAD DEPARTED AND WERE CLBING THROUGH 10500 FT AND WOULD LIKE OUR CLRNC. THE CTLR CAME BACK WITH 'CLRED AS FILED TO MSP; CLB AND MAINTAIN 8000 FT; THEN ISSUED A SQUAWK CODE; ATC DEP FREQ; AND 'HOLD FOR RELEASE.' THE COPLT STATED; 'WE ARE ALREADY AIRBORNE.' ATC SAID TO MAINTAIN 8000 FT; SO WE STOPPED CLBING AND HEADED BACK DOWN TO 8000 FT. JUST AS WE GOT TO 8000 FT; WE WERE CLRED TO FL230. THE CTLR THOUGHT WE WERE STILL ON THE GND. I HAD TRIED TO GET OUR CLRNC BEFORE WE LEFT; BUT THE CTLR WAS HANDLING 3-4 FREQS AND WE DID NOT HAVE A GPU; SO I ELECTED TO SHUT THE ACFT DOWN TO CONSERVE BATTERIES FOR OUR START. I FAILED TO LET CLRNC KNOW I WAS DOING THIS. ONCE OUR PAX SHOWED UP; WE STARTED AND THEN TAXIED OUT AND TOOK OFF. THIS; I BELIEVE; LED TO THE CONFUSION; OR MISCOM. IN THE FUTURE; I WILL ADVISE ATC OF WHAT MY INTENTIONS ARE TO KEEP THEM IN THE LOOP; AND NOT FORGET TO CALL BEFORE A DEP ONCE I'VE TRIED TO GET THE CLRNC. FT COLLINS IS A CLASS E ARPT; SO WE DO DEPART VFR ON OCCASION. I BELIEVE THAT MORE CTLRS ON DUTY DURING BUSY TIMES WOULD ALSO HELP COMS. MONITORING 3-4 FREQS IS NOT ALWAYS IN THE BEST INTEREST OF SAFETY.