They won't tell you - for security reasons! The other clever bit is, most of the phone numbers at airports send you off to recorded message systems - just to make sure that you don't speak to anyone. So the best method is to call your handling agent and get them to transfer you the prats in the airport's office. Alternatively, you could let your aircraft block a stand or taxiway and let them call you... Choking up their apron gets their attention pretty quickly.
PM