You guys make me unsure, but I was always told that I could use first and last 2 letters of the callsign.
So, N100AB would become November Alpha Bravo.... and I ( and most of my collegues ) use it that way. Never heard a complaint...
BTW, pilots have been giving me callsigns like that themselves... the Beluga transporters from Hamburg/Toulouse - wherever always call in with Beluga Tango Bravo ( having FGSTB as callsign )...