Saigor
You may be correct, but from my experience it doesn't stop Luftie pilots complying with US conventions.
That is, they manage to say Lufthansa twelve thirtyfour heavy when in US airspace, rather than the Lufthansa one two three four when in European airspace.
AFAIK, you should only start the transmission with the callsign when you wish to gain the aircraft's attention, and not when replying to an instruction/request.