not known for my in-depth knowledge of the books, however;
full callsigns should be used on first contact and thereafter abbreviated callsigns may be used. however, not all callsigns can be abbreviated.
e.gs
GEHAV abbreviates to G-AV
N234XYX abbreviates to N-XYZ
but
BMA12ER remains BMA12ER for the entire flight.
there is a bit (i think) about omitting callsigns for non-executive instruction transmissions if, after the initial transmissions, the same two people carry on a conversation regarding, say, a slot time or some such.
hope that's helpful (more to the point i hope i'm not going to have my knowledge shown to be too deficient!)