RWY INCURSION. UNAUTH RWY ENTRY.

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UPON TAXIING TO 20C; WE WERE ADVISED BY COMPANY OPS TO CONTACT DISPATCH. SINCE THE CONVERSATION WAS NOT COMPLETE AS I NEARED THE RWY; I PARKED IN THE 'HOLD' AREA AND ADVISED 'GND' WHAT WE WERE DOING. THEY ACKNOWLEDGED AND ASKED US TO CONTACT THEM WHEN READY. WHEN WE DID THAT; GND CLRED US TO 20C. AS I TAXIED UP TO THE RWY; THE FO WAS GIVING THE PRE-DEP BRIEFING TO THE PAX. I CHKED 'FINAL' AND TAXIED DIRECTLY ONTO THE RWY THINKING I HAD BEEN CLRED ONTO THE RWY. THE FO LOOKED A BIT STARTLED; BUT CONTINUED HIS BRIEFING; THINKING I HAD RECEIVED CLRNC. I DON'T REMEMBER IF WE HAD SWITCHED THE RADIO TO 'TWR' YET. I HAD ONLY JUST LINED UP WITH THE RWY AND SET THE BRAKE WHEN TWR CALLED AND ADVISED US THAT THERE WAS A TXWY JUST AHEAD; TO PLEASE TAXI OFF THE RWY SINCE THERE WAS AN ACFT ON FINAL. THIS I DID; AND A COUPLE OF MINS LATER; AN SMA LANDED. CAUSE -- I HAD BECOME PREOCCUPIED WITH THE RESULTS OF THE CONVERSATION WITH DISPATCH; THOUGH I DID NOT PARTICIPATE -- AND HAD ACCEPTED IN MY OWN MIND THAT I HAD BEEN CLRED 'ONTO' THE RWY. HAD I LOCATED THE ACFT ON FINAL; I WOULD HAVE STOPPED. CONTRIBUTING FACTOR -- WX AT DEST -- REASON FOR THE CONVERSATION WITH DISPATCH. ALSO; WE WERE LATE GETTING AWAY FROM THE BLOCKS DUE TO LATE BAGS. CORRECTIVE ACTION -- INCREASED INTENSITY ON MY PART. THIS IS THE ONLY TIME I HAVE EVER TAKEN A RWY WITHOUT CLRNC.