If you want to make a call you have to be at some WIFI point and go to the GrandCentral website or mobile grandcentral website (m.grandcentral.com which isn't flash heavy) and then initate the call from there. It then will call whatever phone you selected (say Gizmo) and if you are running Gizmo or the sipphone # then it'll call the n800 as well as any Gizmo that is logged into that account. To sign up for Gizmo just go to www.gizmoproject.com

loughmillermedia, to connect Gizmo to GrandCentral you just need to add the Gizmo # into Grand Central. When you make a call from the GrandCentral website it will ask you which phones (out of the #s you've listed) to call. For example, for me it's my home phone, Gizmo, or cell phone. Just choose Gizmo after you choose the phone # to dial, and then it'll ring whatever computers have Gizmo Project open. Or the sipphone # (if you are using the built in client in the n800 [not Gizmo] with the sipphone #.

To receive phone calls, you can set up GrandCentral in settings to dial all phones, certain phones (Say, only Gizmo). And when people call your Grand Central #, Grand Central will forward the call to say your n800 running Gizmo or the sipphone service.