Overstress
The publication that you are looking for (in the UK) is CAP 413 sect. 2.7.2.2
My copy is 1994 so it may be a bit out of date
Get 'em to heaven and back has covered it pretty well.
Callsign abbreviation may only be instigated by the ground station.
CAP 413 confirms that operating agency/flight ident must never be abbreviated i.e. SPEEDBIRD 725