If it really is a SIP-based service, it should work with Gizmo as a client on the N800. Just download and try, Gizmo worked with any SIP-provider I tested.

Nokia also promised native SIP-support for 770/N800 looong ago (integrating the Sofia-SIP-library into the ITOS), but I'm not sure if they're still committed to it now they signed a contract with a proprietary VoIP service provider.