I think what you're looking for would be some form of SIP proxy (for lack of a more intelligent term). Every IP (Internet) to phone service I've seen requires you to connect to their server (Yahoo Voice, Nortel, etc..). I don't think there are many (if any) "generic" SIP servers that allow connections from anyone or anywhere. No one would pay for VOIP services.
I could be wrong; probably am, but that's my 2c worth.