AN ACR FLC HAS PROBS WITH CALL SIGN CONFUSION AND TAXIES ONTO THE ACTIVE RWY WITHOUT CLRNC AT SJC; CA.

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WHILE WAITING #1 FOR TKOF AT SJC RWY 30L; WE WATCHED SEVERAL ARR ACFT FOR SPACING AND ANTICIPATED CLRNC ONTO THE RWY. I HEARD WHAT I THOUGHT WAS OUR CALL SIGN; 'CLRED INTO POS AND HOLD.' I ACKNOWLEDGED TWR'S CALL AND WE TAXIED ONTO THE RWY FROM TXWY B1. AS WE PULLED INTO POS; I SAW A PVT BEECH 1900 ALSO TAXI INTO POS FURTHER DOWN THE RWY FROM TXWY C ON THE W SIDE OF THE ARPT. ABOUT THIS TIME TWR CALLED ASKING IF WE WERE TAXIING ONTO THE RWY AND I ANSWERED; 'YES; WE WERE.' TWR TOLD US TO CLR THE RWY AT TXWY B; WHICH WE DID. THE BEECH 1900 WAS CLRED FOR TKOF; AN ACR LANDED; ANOTHER ACR WAS CLRED FOR TKOF AND THEN WE WERE CLRED FOR TKOF. THERE WAS NO CONFLICT INBOUND TFC THAT I AM AWARE OF. WE WERE ON AND OFF THE RWY IN UNDER A MINUTE AND SAFETY WAS NEVER AN ISSUE BUT MY ANTICIPATION OF BEING CLRED ONTO THE RWY AND OUR RECENT CHANGE FROM 3 DIGIT CALL SIGNS TO 4 DIGIT CALL SIGNS CAUSED ME TO ANSWER TO ANOTHER ACFT'S CLRNC AND TAXI ONTO AN ACTIVE RWY WITHOUT CLRNC. TWR'S QUERY WAS MY FIRST INDICATION OF WHAT HAD HAPPENED. LISTENING MORE CLOSELY TO RADIO XMISSION AND QUESTIONING THE CTLING AGENCY IF ANY DOUBT ABOUT A RADIO CALL WOULD HAVE PREVENTED THIS.