C172 PLT; CONDUCTING IFR TRAINING; MISUNDERSTANDS CTLR'S CLRNC INSTRUCTIONS AND DEPARTS AN UNCTLED ARPT BEFORE BEING RELEASED.

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TODAY; I DEPARTED IFR FROM AN UNCTLED ARPT FOR PRACTICE INST APCHS. DUE TO THE AIRSPACE RESTRS CURRENTLY IN PLACE (WTC AND PENTAGON); WE USED A CELL PHONE (AS WE HELD SHORT OF THE RWY) TO CALL MINNEAPOLIS APCH FOR OUR IFR CLRNC (NORMALLY; WE DEPART VFR AND RECEIVE CLRNC AIRBORNE). THE PERSON I SPOKE TO INDICATED THAT SHE COULD NOT RELEASE US YET AS SHE HAD TFC ON A NEARBY APCH. HOWEVER SHE SAID THAT SHE COULD GIVE US OUR CLRNC AND TO HOLD FOR RELEASE. I COPIED OUR CLRNC AND THEN SHE STARTED GIVING US A CLRNC VOID TIME. AS SHE STARTED TO GIVE ME THE CLRNC VOID TIME; A NEARBY B25 STARTED ITS RUN-UP CREATING A GREAT DEAL OF NOISE AND INTERFERENCE TO MY PHONE. THEREFORE I HAD TO ASK THE CTLR TO READ THE VOID TIMES 3 TIMES. I REPEATED BACK TO HER WHAT I HAD HEARD (VALID AT XA31Z VOID AT XA33Z). I SAID THAT I WOULD RPT AIRBORNE. AT THIS POINT WE DEPARTED RWY 16 (SGS) AND CONTACTED MINNEAPOLIS APCH ON 121.2 AT WHICH TIME THE CTLR INDICATED THAT WE WERE AIRBORNE 2 MINS EARLY. THIS WAS A SURPRISE TO ME AS I HAD NEVER HEARD A CURRENT TIME ISSUED WHEN I RECEIVED MY VOID TIME. I INCORRECTLY ASSUMED THAT SHE HAD GIVEN US A 2 MIN WINDOW UPON WHICH TO DEPART IMMEDIATELY. ALSO; WHEN ON THE GND ON MY CELL PHONE; I MENTIONED THAT I COULD NOT HEAR WHAT WAS BEING SAID DUE TO THE NOISE OF THE B25 BEHIND US. WHEN I READ BACK WHAT I HEARD; THERE WAS NOT ANY MENTION OF THE CURRENT TIME TO CLARIFY AND POINT OUT THAT THE CLRNC WAS NOT YET VALID. CONTRIBUTING FACTORS: NOISE FROM B25; LACK OF UNDERSTANDING BTWN MYSELF AND CTLR; AND BUSY AIRSPACE AND FLT RESTRS.