AN SF-340 CREW STARTED ITS TKOF ROLL BEFORE BEING CLRED. TKOF ABORTED WHEN FO REALIZED THAT A MISTAKE MAY HAVE BEEN MADE. ERROR ADMITTED.

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CAPT CALLED FOR 'BEFORE TKOF' CHKLIST WHILE APCHING RWY. I BEGAN CHKLIST BY BRIEFING PAX OVER THE PA SYS WHICH CUTS ME OUT FROM HEARING ATC INSTRUCTIONS. WHEN FINISHED WITH MY BRIEFING; CAPT SAID 'HERE'S THE SQUAWK CODE' AND THEN CALLED FOR 'BOXED ITEMS' (FINAL BEFORE TKOF CHKLIST ITEMS). I COMPLETED THE CHKLIST WHILE CAPT TAXIED ON TO RWY. HE THEN ASKED ME IF I WAS READY TO GO AND I RESPONDED BY SAYING 'YES.' CAPT ADVANCED PWR LEVERS FOR TKOF AND WE BEGAN THE TKOF ROLL. SHORTLY THEREAFTER I SAID 'WE HAVE BEEN CLRED FOR TKOF?' AT WHICH TIME TWR SAID OVER THE RADIO 'ACR X; POS AND HOLD!' UPON HEARING THIS; WE ABORTED TKOF AND THEN CONSEQUENTLY RETURNED FOR TKOF. PROB AROSE WHILE I WAS BRIEFING PAX OVER PA. I COULD NOT HEAR ATC SPEAKING TO CAPT AND WHEN I CAME BACK ON; I WAS OUT OF THE LOOP TEMPORARILY UNTIL I ASKED CAPT 'WE HAVE BEEN CLRED FOR TKOF?' ON THE ROLL. THEN WE ABORTED. SUPPLEMENTAL INFO FROM ACN 348645: I MISREAD POS AND HOLD CLRNC FOR TKOF CLRNC. I STARTED TKOF ROLL; THEN QUICKLY REALIZED ERROR AND ABORTED TKOF; GOT CLRNC FROM TWR FOR A 180 DEG TURN TO TAXI BACK FOR TKOF AGAIN.