Using the correct callsign for the ATC unit ensures that the ATCO is confident that the pilot understands the level of service that is being provided. This is really more important in the Tower, where in the most extreme cases pilots have been known to take off or land without clearance, under the impression that the unit is FISO or Air-Ground rather than ATC. ("xyz information" rather than "xyz tower"). Any unit that you're contacting will (or should) correct you on initial contact if the callsign is wrong, and that's the callsign you should subsequently use. Many Airports only operate Radar for limited amount of time in any 24hr period and their callsign will change as appropriate. (Hence the possibility of confusion with neighboring Units). The level of service that you require is not relevant to the callsign, however it should be noted that just because you're speaking to a Radar unit it doesn't mean you'll get traffic information. (Unless you require a Radar service, but that's another matter)
Long winded perhaps, but hope it helps
Last edited by zounds; 2nd December 2008 at 18:02.