Go to the main FSX directory and search for a .cfg file named "airlines". There are the callsigns ATC use, so add "Speed Bird" for BA (Note: "Speedbird" won't work), "Japan Air" for JAL, etc. Do not remove the original callsign "British Airways" or "Japan Airlines" from the .cfg until you select the new custom callsign at least once with your flight or you will get the error: "British AirwaysSpeed Bird" or Japan AirlinesJapan Air" even if the are not there anymore.

Good luck