I did some further research in the Gizmo forums regarding the "authentication failed" problem I reported earlier today (and confirmed by ShyBoy). A Gizmo user troubleshooting a similar problem last year on another device reported that the user name in account setup should contain your SIP Number and not your Gizmo Name.

In the N800 Internet Call "Account setup: User Name" dialog, I replaced my Gizmo Name with my SIP Number (for example: 17471234567@proxy01.sipphone.com). That did the trick and SIP is now "connected" on my N800. My internet calling was working yesterday with myGizmoName@proxy01.sipphone.com, so something must have changed in their system. Maybe they will reply to Shyboy's ticket with an explanation.