Thanks for the reply and link FD, but it doesn't quite answer my question (althogh, strangely enough, the airline in question isn't listed in the document you linked, but I can get it's ICAO info through google).
Assume an airline; "Flaky Air", with ICAO code YYY (yes I know that's reserved) and callsign "WingandaprAir". My (all be it limited) understanding is that on all flights operated by the airline (with the possible execption being wet-leases to another operator) in all radio communications with ATC the aircraft would identify itself as "WingandaprAir <Flight Designator>". So, under what circumstances would they delibertily identify the flight as "YYY <Flight Designator>"?
The only reason I asks is I overheard a flight using its ICAO code rather than callsign in all ATC communications and I'm fairly sure it was intentional, as at one stage the pilot communicating with ATC initially used the airlines callsign, then restarted the messages using the ICAO code. Before anyone gets uppity, it's not a conspiracy, I'm not questioning the abilities or motives of those involved. Its nothing more the idle curiosity (and my choice of hypotetical airline name and callsign is not intended to infer any view on the airline or individuals ivolved, it is intended to be light hearted).
JAS