Pilots are not generally allowed to choose their callsigns, although whichever callsigns they get could be very well be attibuted to their own inadvertent choices...

Let me try to explain...
For those who arent sure, callsigns are a series of unique nicknames used to identify certain members of the military. Most of the time the only dudes who have a callsign are pilots, but its not rare for other members of the service like the Navy or Army to have a callsign too, especially SpecOps units.
As a pilot in the RSAF (and most of the Air Forces Around the World), you get a callsign based on something which you have done...usually its something generally bad or embarassing, one u screwed up from, or can just be a blatant mockery of your name. This callsign can be given to you by the Base Commander or even by your pals, and recognized officially.
Examples:
Lt. Mark "Soup" Campbell
Lt. Paul "Orr" Gasm
and so on...
Most of them are really funny when u manage to figure it out how it came about, and more often than not, there's always a history or very funny tale behind those callsigns (ask any pilot and he'll tell u some very interesting stories about his callsign)
As a rule of thumb, one should always remember this:
1) If you're a pilot and dont have a callsign yet, one will definitely be assigned to you.
2) You probably wont like it.
3) If you moan and groan too much about 1) and 2), you'll get a new callsign you'll like even less

Hope this helps..