It's not just dependent on your connections... both my wife's N9 and mine have identical internet connections; wifi at home and the same 3G provider when out and about. Her phone never seems to register properly with the SIP server; she can generally make outgoing calls, but never gets incoming calls, including from me (they just end with a connection errror). However my N9 never seems to have a problem with the SIP server (same account; same server, etc) and I can pretty much rely on being able to make and receive calls whenever.

We do both have issues with Google Chat though. On both of our phones it usually fails to connect, with the message "Invalid password", which is rather frustrating, because there is nothing wrong with our passwords.