This is a complex issue. We don’t want to censor callsigns, but we also can’t let people choose whatever they want without restriction (because your callsign is visible to other players so it affects them, and also because you might be increasing the game’s age rating).
So I think this will be handled as follows: The terms of use of the game will specify that your callsign (and anything else you enter yourself, such as your spacecraft names or your squadron names) must not be offensive, objectionable, sexual, etc. This will be coupled with a mechanism to report offensive callsigns from within the game. If enough people complain, your callsign will be removed (or at least, made invisible to other players).