B747-200 FLC RESPONDED TO ANOTHER ACFT'S CLRNC AFTER A LONG WAIT TO CROSS ACTIVE RWY FOR PARKING. THE 3 CREW MEMBERS BELIEVED THAT THEY HEARD THEIR ACFT CALL SIGN TO CROSS RWY.

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WE LANDED ON RWY 27L AT XA10L. AFTER LNDG; WE ROLLED TO THE END AND WERE INSTRUCTED BY TWR TO EXIT R ON TXWY M1; TAXI BACK TO TXWY K; HOLD SHORT OF RWY 27L AND REMAIN ON TWR FREQ. WE COMPLIED AND WAITED PATIENTLY FOR CLRNC TO CROSS RWY 27L AND TAXI TO THE RAMP. NUMEROUS ACFT LANDED AND WERE CLRING AT TXWY T; WHICH IS PRIOR TO TXWY K. AT APPROX XA15; AS AN ACFT WAS CLRING AT TXWY T WITH ANOTHER ACFT ON APPROX A 3 MI FINAL; THE TWR RADIOED 'ACFT X CLRED STRAIGHT ACROSS.' I VISUALLY CLRED THE RWY. NEITHER THE ACFT CLRING AT TXWY T NOR THE ACFT ON FINAL WAS A FACTOR AND I STATED 'CLRED TO CROSS.' TWR THEN CALLED; 'ACFT X; ARE YOU ON?' AS THE CAPT RESPONDED WE WERE AND THAT WE WERE XING THE RWY; I STARTED TAXIING ACROSS THE RWY. ONCE WE WERE ACROSS RWY 27L; TWR CALLED BACK AND SAID 'ACFT X; YOU WERE NOT CLRED TO CROSS.' THE CAPT RESPONDED WE HAD HEARD 'ACFT X CLRED STRAIGHT ACROSS.' TWR RESPONDED THAT WE TOOK SOMEONE ELSE'S CALL SIGN; ANOTHER SIMILAR CALL SIGN. WE ALL HAD A BIG SINKING FEELING -- WE WERE CLRED; WEREN'T WE? THE 3 PLTS AND AN FE ON THE FLT DECK ALL RECONFIRMED AMONGST OURSELVES THAT WE ALL UNDERSTOOD WE WERE CLRED TO CROSS. THERE WAS NEVER A CONFLICT WITH ANY ACFT. THE TAXIING ACFT WAS CLRING AT TXWY T AND THE ACFT ON SHORT FINAL WAS NOT SENT AROUND AND LANDED NORMALLY. SO HOW DID WE MISTAKE 'ACFT X CLRED STRAIGHT ACROSS' FOR OUR CLRNC? WE UNDERSTOOD WE WERE CLRED TO CROSS; AND WITH AN ACFT ON SHORT FINAL WE DIDN'T WANT TO DELAY. AT BUSY ARPTS; THEY WANT YOU TO BE PROMPT. IN HINDSIGHT; MAYBE WE SHOULD HAVE WAITED FOR THE TWR TO RECONFIRM THE CLRNC; BUT THEN WE ALL UNDERSTOOD WE WERE CLRED ACROSS. WAS THE PHRASEOLOGY CORRECT? PROBABLY NOT. WHILE I CAN'T FIND THE SPECIFIC VERBIAGE IN AIM; 7110.65 INSTRUCTS CTLRS TO STATE 'CROSS RWY XX' WHEN XING AN ACFT ACROSS. THE CLRNC 'ACFT X CLRED STRAIGHT ACROSS' DID NOT INCLUDE THE RWY. SO WE SHOULD HAVE RECONFIRMED THE CLRNC BEFORE XING. LESSON LEARNED!