Dunno if that is possible on the bus, but on boeings you can preprogram the airline identifier (three letter icao airline code) so all you need to enter is the rest of the callsign. Might be the case for the AF guys to save them one little step. But then it is allways possible to simply re-enter the callsign on boeings at anytime, in flight as well.
ATC usually complains very very fast indeed now if you enter the wrong callsign, in europe they partways use mode-S callsign allocation instead of squawk codes, even in france, so a correct callsign is a must.