A friend of mine of days past who was (and probably still is) a controller
told me that they always want the whole tail number on first contact -
records and tapes and all that, dontchaknow. Once that is done, if ATC
abbreviates your tail no. in a transmission to you, you're then free to do
the same. Helps un-clutter the airwaves with the use of fewer syllables, I
guess. If ATC keeps using your full no. you should too -- it's likely
there's another tail no. in the area similar to yours.

Same thing goes for communications with a tower.

Dan Schaefer