FLC OF AN HVT TOOK OFF WITHOUT TKOF CLRNC.

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AS WE WERE TAXIING FOR DEP AT BRUSSELS NATIONAL THE TWR CTLR CLRED US TO 'LINE UP RWY 25R.' AS WE LINED UP ON RWY 25R THE CAPT BROUGHT THE THRUST LEVERS UP AND CALLED OUT 'SET TKOF THRUST.' THE FO COMPLIED AND WE ACCELERATED DOWN THE RWY. AS WE LIFTED OFF; BRUSSELS TWR CALLED US AND REQUESTED THAT NEXT TIME WE WAIT FOR OUR TKOF CLRNC. THE CAPT AND FO HAD ASSUMED WE WERE CLRED FOR TKOF. I WAS PERFORMING IOE INSTRUCTION WHILE OCCUPYING ONE OF THE JUMPSEATS AND HAD EXPERIENCED PROBS WITH MY HEADSET WHILE TAXIING OUT. I SWITCHED HEADPHONE JACKS JUST BEFORE TKOF AND I ASSUMED THAT I HAD MISSED THE CLRNC. THIS IS A CLASSIC CASE OF 4 RELATIVELY EXPERIENCED PLTS HEARING (OR ASSUMING) WHAT THEY EXPECTED TO HEAR - NOT WHAT WAS ACTUALLY SAID. I ALSO FEEL THAT THE PHRASEOLOGY 'CLRED TO LINE UP RWY 25R' IS EASILY CONFUSED WITH 'CLRED TO TKOF RWY 25R.' 'TAXI INTO POS AND HOLD' IS A CLRNC THAT IS NOT SO EASILY CONFUSED; ESPECIALLY FOR UNITED STATES PLTS. IN THE FUTURE I WILL QUESTION A CLRNC IF I DID NOT HEAR OR UNDERSTAND ALL OF IT. IT IS DANGEROUS TO ASSUME THAT ALL OF THE OTHER COCKPIT CREW HEARD AND UNDERSTAND THE CLRNC.