Inside nations.ini (extract it from flight folder from your installation directory) you can see under each nation the line PilotNameList=NamesXXX.lst. This line tells to game where to look for the available callsigns for each air force. Of course, stock game includes only US call signs, so all air forces use it by default.
If you want to create a callsign which will be used all the time, I guess you have to delete the other ones inside callsignus.lst and keep only one. But if you do this, every flight should use the same name, so you have to create an another callsignXX.lst
Create an .lst file, name it as you like, drop it into flight folder (of your mods directory), open it and add these lines:
[DefaultCallsign]
XXX <--- Put here the name you like
[GroundCallsign]
Delta 47
Echo 77
Gulf 88
Hotel 45
Kilo 52
Mike 83
November 90
Papa 34
Sierra 68
Tango 61
Victor 14
Zulu 29
[ReconCallsign]
Kodak
[FACCallsign]
Snoopy
Probably it's gonna work but you need to edit the subtitles.srt and change the .wav files cause you will hear the stock ones while in the screen you will see the one you selected. After this don't forget to assign the nation with your new callsign.lst. (described at the top of the post)