SIMILAR SOUNDING CALL SIGNS CREATE PROB WITH UNAUTH RWY XING.

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WE WERE ACR A XYZ LANDED RWY 13R AND CLRED TO TAXI ON A TO 32 TO HOLD SHORT OF RWY 18R AND MONITOR TWR 124.15. ACR B XYZ WAS ON TWR FREQ LNDG 18R. APCH HAD ISSUED US A WARNING OF IDENTICAL CALL SIGN NUMBERS. WHILE HOLDING SHORT; I WENT OFF FREQ TO CONFIRM OUR GATE -- CAPT STAYED ON TWR FREQ. I CAME BACK ON AND WE CONTINUED TO HOLD. TWR FREQ WAS VERY BUSY -NONSTOP INSTRUCTIONS. I HEARD; 'ACR A XYZ CROSS 18R CONTACT GND 121.8.' THE CAPT TAXIED ACROSS RWY 18R AS I ACKNOWLEDGED 'ACR A XYZ.' I THEN REALIZED THE INSTRUCTIONS WERE PROBABLY WRONG BECAUSE WE NEEDED TO STAY ON TWR 124.15 TO CROSS 18L. TWR CALLED TO SEE IF WE WERE STILL ON FREQ. I SAID; 'ROGER.' TWR SAID THEY HAD INTENDED FOR ACR B XYZ TO CROSS. ACR B; HOLDING ON TXWY 29 BTWN 18R AND 18L; HAD NOT RESPONDED OR MOVED. THE CAPT AND I AGREED WE HAD HEARD 'ACR A XYZ.' TWR FREQ WAS VERY BUSY -- NONSTOP TWR TALK WITH QUICK ACKNOWLEDGMENTS BY ACFT. WE CALLED TWR ON THE LANDLINE AT THEIR REQUEST. THEY SAID THE CTLR AND 2 SUPVRS AGREED THE CALL WAS ACR B XYZ NOT ACR A XYZ. THEY LATER SAID THE TWR TAPE RECORDING CONFIRMED THIS. 1) WHY DID NO ONE HEAR 'ACR A' RESPOND VERSUS 'ACR B'? 2) A REVIEW OF THE TAPE WILL REVEAL LOUD BACKGND NOISE IN THE TWR -- OTHER CTLRS; CALL SIGNS; AND CTLRS TALKING TO EACH OTHER; SUPVRS INCLUDING DISCUSSION OF THE 2 ACR B AND ACR A XYZ CALL SIGNS; ALL ON TWR 124.15 FREQ. 3) AS FO I SHOULD HAVE REMAINED ON FREQ AND NOT CONTACTED THE GATE EVEN THOUGH CTLR PRESSURE YOU TO HAVE YOUR GATE INFO AS SOON AS YOU TALK TO GND. 4) AS FO; I SHOULD HAVE QUESTIONED THE TAXI CLRNC PART ABOUT CONTACTING GND. IF I HAD BEEN 100 PERCENT IN THE LOOP; I WOULD HAVE CAUGHT THIS AS INAPPROPRIATE FOR OUR POS. 5) TWR SUGGESTED IN CONVERSATION WITH CAPT ROOM FOR IMPROVEMENT BY ALL PARTIES AND SAID THEY DID NOT INTEND TO PURSUE THE INCIDENT FURTHER WITH ACR A XYZ.