MATS Part 1 says that once satisfactory two way communication with an aircraft has been established controllers are permitted to abbreviate the callsign. I tend to use the full callsign during my initial response to a 'freecall' simply in order to check that I have clearly understood exactly what the full callsign was. After I have 'established satisfactory two way communication' I will always abbreviate unless a possibity of callsign confusion exists. The only perculiarity at LTCC that I am aware of is a consession which allows the ground station not to use it's callsign on intitial contact in certain circumstances.