B767 FLT CREW; UNFAMILIAR WITH SPECIAL IAD TAXI PROCS; TAKE CLRNC ISSUED TO ANOTHER ACR AND TAXI UNTIL CHALLENGED BY GND CTLR.

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BOTH THE FO AND MYSELF HAVE VERY LITTLE EXPERIENCE FLYING IN AND OUT OF IAD. JUST PRIOR TO ENG START; WE REALIZED THAT THEY USE A DIFFERENT PROC. WE PULLED OUT THE BULLETIN AND REVIEWED THE PROC. WE HAD JUST FLOWN IN FROM BOS AND DATA LINK DELIVERY OF TAXI CLRNC WAS NOT USED ON TAXI IN. WE PUSHED BACK FROM GATE FOR A W TAXI. NEITHER OF US LIKED THE HEADS DOWN ACARS WORK WHILE STARTING ENG AND TAXIING. WE PULLED UP BEHIND AN RJ ON SPOT X. WE WERE HAVING SOME TROUBLE GETTING THE ACARS MESSAGE AND I HEARD THE TAXI CLRNC FOR THE RJ AS I TURNED UP THE VOLUME ON GND. THEN FO READ THE ACARS MESSAGE AND I HEARD THE TAXI CLRNC AND I BEGAN TO FOLLOW IT. I MADE THE TURN ONTO TXWY Z AND STOPPED ABEAM THE END OF THE TERMINAL AS ATC WAS QUERYING US. THE CONFUSION WAS ALSO COMPOUNDED BY A RWY CHANGE; SO WE WERE PUTTING THE NEW ONE IN THE FMC RIGHT BEFORE THE HOLD SPOT; EVEN THOUGH WE REVIEWED THE PROC. I MISREAD THE ACARS CLRNC THAT SAID 'EXPECTED TAXI CLR.' IT WAS A CLR DAY WITH VERY LITTLE TFC MOVING AROUND THE ARPT. ATC WANTED TO KNOW WHAT THE ACARS MESSAGE HAD SAID. APPARENTLY THEY DID NOT SEND IT. SUPPLEMENTAL INFO FROM ACN 573716: WE WERE COMPLYING WITH THE PUSHBACK AND TAXI INSTRUCTIONS AS PER BULLETINS FROM GATE. WE ENTERED 'PR' INTO RPT MISC PAGE AND PUSHED OFF THE GATE AFTER ACARS MESSAGE TO PUSH CLRNC. WE SENT 'PR' RPT AGAIN AND GOT MESSAGE TO TAXI TO SPOT XX. WE ENTERED 'TR' RPT AND COMPLIED WITH WHAT WE THOUGHT WAS TAXI CLRNC MESSAGE TO RWY 30. GND CALLED US AND ASKED WHY WE WERE TAXIING. WE TOLD THEM WE WERE COMPLYING WITH THE ACARS TAXI CLRNC. WE BOTH CHKED THE ACARS AND NOTICED IT WAS AN EXPECT CLRNC. CRM. WE WERE READING THE BULLETIN AND PERFORMING MULTIPLE ACARS CHALLENGE AND RESPONSE PROCS WHILE HEADS DOWN AND TAXIING. WE THUS MISREAD THE PROC AND THOUGHT WE WERE GIVEN TAXI CLRNC. THE PROC COULD BE A GOOD ONE IF STANDARDIZED AT EVERY ARPT AND SIMPLIFIED TO 1-2 ACARS RPTS INSTEAD OF 4.