I'm an Air Traffic Controller in Lisbon Approach/TMA and dur to the proximity of several small aerodromes near Lisbon Airport we have a 30 NM Radius area where the FIS (even on G Airspace) is Approach responsability.
On first contact I expect to ear full (registration) Callsign and on my first reply I also use the full callsign.
Only after first contact I expoect to ear and use reduced callsign.
However, and this is not a rule but a common practice, if ATC uses complete callsign, pilots tend to reply using also the complete callsign - it seems they know we have a motif for not using the reduced callsign and so we don't need to ask for pilots to do it when similar reduced callsigns are in the frequency.
Did it help ?