ACR CREW MISHEARS XING CLRNC AND CROSSES HOLD SHORT LINE.

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WE HAD JUST LANDED ON RWY 4R AT XYZ. WE WERE HOLDING SHORT OF RWY 4L WAITING TO BE CLRED ACROSS. WE THEN HEARD OTHER ACFT BEING CLRED TO CROSS RWY 4L; BUT THOSE ACFT WERE S OF US. THE FO THOUGHT THAT WE WERE IN FRONT OF THESE ACFT ON THE TXWY; SO HE CALLED THE TWR AND ASKED IF WE WERE CLRED TO CROSS RWY 4L. THE FREQ WAS BUSY; BUT WE THOUGHT WE HEARD THE TWR REPLY 'YES YOU ARE' AND SO THE FO READ BACK 'CLRED TO CROSS RWY 4L.' AS OUR NOSE CROSSED THE HOLD SHORT LINE; THE TWR CALLED US AND TOLD US TO HOLD SHORT. WE STOPPED WITH OUR NOSE ROUGHLY 8 FT OVER THE HOLD SHORT LINE. CONFUSION ON THE FREQ AND CONFUSION ON THE PART OF OUR CREW LED TO THE MISUNDERSTANDING. OUR READBACK AND THE QUICK REACTION OF THE CTLR PREVENTED THIS MISUNDERSTANDING FROM BECOMING A VERY BAD SIT. CLOSER ATTN TO THE FREQ AND TO THE POS OF THE OTHER ACFT WOULD HAVE GREATLY HELPED OUR CREW FROM MAKING THIS MISTAKE. SUPPLEMENTAL INFO FROM ACN 452303: FIX: BACK TO BASICS XYZ MAY GET FREQ SATURATED; BUT I STILL HAVE TO GET WHOLE CLRNC INCLUDING CALL SIGN. SUPPLEMENTAL INFO FROM ACN 452285: THIS MISCOM COULD HAVE BEEN PREVENTED IF I HAD WAITED FOR THE PROPER CLRNC TO CROSS RWY 4L. I BELIEVE THE NEARLY 12 HR DUTY DAY WAS A FACTOR.