ACR X RWY INCURSION CAUSED B-727 GAR.

Narrative

ACR X APCHED THE #1 POS AT THE APCH END OF RWY 12R AND SWITCHED OVER TO TWR FREQ (118.5). WE HEARD TWR SAY CLRED INTO POS AND HOLD RWY 12R. SINCE WE WERE #1 FOR TKOF AT THE APCH END; WE ASSUMED THE CALL WAS FOR US. WE ACKNOWLEDGED THE XMISSION AND RESPONDED THAT WE WERE GOING INTO POS AND HOLD RWY 12. TO CONFIRM THE INSTRUCTION WAS FOR US WE ASKED TWR TO CONFIRM POS AND HOLD. WHILE MAKING THIS RADIO CALL WE PARTIALLY BLOCKED TWR'S NEXT XMISSION WHICH WAS 'THERE'S A B-727 ON A 2 MI FINAL CLRED FOR AN IMMEDIATE TKOF.' I STILL THOUGHT TWR WAS TALKING TO US UNTIL I LOOKED DOWN THE RWY AND SAW A LIGHT TWIN ON TKOF ROLL. AT THAT TIME I ADVISED TWR WE THOUGHT THE INSTRUCTION WERE FOR US AND THAT WE HAD ENTERED THE RWY EDGE. HE SAID 'NO HE HAD NOT CLRED US ON THE RWY BUT THANKED US FOR ADVISING HIM;' HE THEN SENT THE B-727 ON FINAL AROUND. I BELIEVE THE CTLR'S RUSH TO GET ACFT AIRBORNE; AND NOT USING FULL CALL SIGNS AND HIS FAILURE TO ACKNOWLEDGE OUR XMISSIONS CONTRIBUTED TO THE CONFUSION RESULTING IN OUR ENTERING ONTO THE RWY. ADDITIONALLY; BECAUSE WE WERE #1 FOR TKOF AND HEARD THE STANDARD TERMINOLOGY ASSOCIATED WITH BEING #1 FOR TKOF; WE HEARD WHAT WE ANTICIPATED HEARING IN THAT SIT.