If you can get SP1 to work with a particular provider, but not SP2 even when you use the exact same settings, then I'd check to see if any default settings are different — for example, by default X_UserAgentPort is set to port 5060 for SP1 and 5061 for SP2, but if you are using SP1 and SP2 with different providers then there's no reason that the SP2 port can't also be 5060 (it only has to be different if both SP's go to the same provider and switch). Changing that setting in SP2 might fix your problem, assuming you haven't tried that already.