B757 CREW BELIEVED THEY WERE CLRED INTO POS ON RWY 25R AT LAX. CTLR QUESTIONED WHETHER THEY WERE.

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HOLDING SHORT FULL LENGTH 25R AT LAX. FO HEARD AND RESPONDED TO TAXI INTO POS AND HOLD CLRNC. AT THAT TIME DURING BEFORE TKOF CHKLIST I WAS CONDUCTING 'DEPARTURE BRIEFING' AND ONLY HEARD THE NUMBER PART OF OUR CALLSIGN. AS WE PROCEEDED ONTO RWY TWR ASKED IF WE WERE STILL HOLDING SHORT. TWR; AT THE SAME TIME WAS CLRING AN ACR Y FLT ACROSS RWY AT INTXN FOXTROT AND AN ACR Z FLT INTO POSITION ALSO AT TXWY FOXTROT. TWO MAIN FACTORS CONTRIBUTED: 1) I SHOULD NOT MOVE ACFT UNTIL TOTALLY CERTAIN I HEAR BOTH PARTS OF CALLSIGN. 2) I SHOULD NOT BE SO INVOLVED IN A BRIEFING THAT I AM NOT LISTENING TO ATC CLEARANCES. NO CONFLICT OCCURRED ON RWY. WE WERE ABLE TO HOLD ON FULL LENGTH WHILE OTHER ACFT DEPARTED FROM INTXN F (A NORMAL PROC). SUPPLEMENTAL INFO FROM ACN 520573: OUR FLT WAS HOLDING SHORT OF THE RWY 25R HOLD SHORT LINE AT TXWY 'B.' WE WERE ACCOMPLISHING THE BEFORE TKOF CHKLIST; WHEN I THOUGHT I HEARD THE FOLLOWING CLRNC; 'ACR X; RWY 25R; TAXI INTO POSITION AND HOLD;' I ANSWERED; 'POSITION AND HOLD RWY 25R; ACR X.' THE CAPT AND I BOTH FELT AT THAT POINT THAT WE HAD ACCEPTED AND ACKNOWLEDGED A VALID CLRNC ONTO THE RWY. THE CTLR DID NOT RESPOND; NOR WAS THERE ANOTHER ACFT WHO ANSWERED THE CLRNC. ADDITIONALLY; WE WERE THE FIRST ACFT IN SEQUENCE TO DEPART FULL-LENGTH ON RWY 25R; THE CLRNC 'MADE SENSE' TO BOTH THE CAPT AND ME. THE CAPT TAXIED ONTO THE RWY AND WE CONTINUED THE CHKLIST. AFTER WE HAD STOPPED IN POS; THE CTLR ASKED US TO CONFIRM THAT WE WERE HOLDING SHORT OF THE RWY. WE RESPONDED THAT WE WERE IN POS AND HAD ACKNOWLEDGED A CLRNC TO DO SO. AT THAT TIME ANOTHER ACFT WAS CLRED TO CROSS RWY 25R AT TXWY 'F' AND PROCEED TO RWY 25L. ALSO; TWO REGIONAL ACFT WERE CLRED TO DEPART AHEAD OF US ON RWY 25R DOWNFIELD AT INTXN 'F.' AT THE TIME OF DEP RWY 25R WAS BEING USED FOR DEPARTURES ONLY. RWY 25L WAS IN USE FOR ARRIVALS. I BELIEVE THAT A BUSY ATC ENVIRONMENT; RAPIDLY ISSUED CLEARANCES; AND INADEQUATE FEEDBACK; BOTH ON OUR PART AND THE CTLR'S; CONTRIBUTED TO THIS INCIDENT. WE SHOULD HAVE BEEN MORE DILIGENT TO CONFIRM OUR CLRNC; AND THE CTLR MIGHT HAVE PREVENTED THE PROB BY RESPONDING TO OUR READBACK.