A B737-300 IS CLRED INTO POS FOR TKOF AND THE TWR CTLR ADVISES THEM THAT THEY HAD ACCEPTED THE CALL SIGN FOR ANOTHER FLT; SAME COMPANY; AT LAX; CA.

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WE WERE ACR X FLT LAX TO SFO. WE HAD A 1 HR 5 MIN DELAY AT THE GATE FOR A MECHANICAL PROB. WHEN WE WERE READY TO TAXI; LAX GND CTL CLRED US TO TAXI TO RWY 25R VIA TXWY B. LAX TWR TOLD US TO TAXI THE FULL LENGTH OF TXWY B TO RWY 25R; THEN MADE A CORRECTION AND TOLD US TO CONTINUE HOLDING SHORT OF TXWY F ON TXWY B. (THERE WERE 2 AIRPLANES AHEAD OF US HOLDING SHORT OF THE RWY AT THE FULL LENGTH OF TXWY B. THE FIRST ONE WAS ANOTHER COMPANY ACFT; A B737-300; AND THE SECOND WAS A BRASILIA ACFT.) THE NEXT XMISSION WE HEARD WAS 'ACR X FOX (TROT) RWY 25R TAXI INTO POS AND HOLD AND KEEP THE DEPARTING BRASILIA IN SIGHT.' WE RESPONDED TO THE CLRNC; CHKED THE FINAL FOR AIRPLANES; AND TAXIED INTO POS ONTO RWY 25R AT INTXN FOXTROT. AS I SET THE BRAKES AWAITING TKOF CLRNC; THE TWR CTLR TOLD US THERE WERE SIMILAR CALL SIGNS ON FREQ (WHICH WE WEREN'T AWARE OF); AND HE SAID HE HAD CLRED OUR FLT AT THE FULL LENGTH INTO POS AND TO HOLD. THE CTLR THEN SAID THAT IT WAS OK AND TO LISTEN MORE CLOSELY FOR SIMILAR CALL SIGNS. WE WERE GIVEN TKOF CLRNC AND THE FLT PROCEEDED UNEVENTFULLY. SOME OF THE PERCEPTIONS WE HAD WERE: THAT AFTER THE TWR HAD CLRED US TO TAXI FULL LENGTH TO RWY 25R; THEN CORRECTED HIMSELF TO SAY HOLD SHORT AT TXWY FOXTROT FOR RWY 25R; WE THOUGHT HE MAY CLR US TO TAKE OFF FROM THE INTXN FOXTROT ON RWY 25R. THEN WHEN THE VERY NEXT XMISSION WAS TAXI INTO POS AND HOLD; THE SIMILAR CALL SIGNS OF ACR X FOX (TROT) AND ACR Y MUST'VE BEEN A CONTRIBUTING FACTOR. WITH THE DELAY WE INCURRED; I ALSO ASSUMED THE CTLR MAY HAVE BEEN GETTING US OUT SOONER TO KEEP OUR FLT PLAN ALIVE. I WILL KEEP AN EVEN MORE DILIGENT WATCH ON RADIO CALL SIGNS. NOTE: I DID CALL LAX TWR ON THE TELEPHONE AND A SUPVR I SPOKE WITH WHO WITNESSED THIS SAID THERE WERE 2 FLTS OF SAME COMPANY; BOTH GOING THE SAME DIRECTION WITH THE SAME SID AND DEST. HE SAID 'NOTHING TO WORRY ABOUT.' THE CTLR MAY HAVE SAID 'FLT ACR X FOX' BECAUSE HE MAY HAVE MIXED UP THE DEP STRIPS.