I can make calls from the Google Voice web site using the N800's factory installed browser (don't know it's name) and the Gizmo client running on the N800. If I try to use the mobile version of the Google Voice web site then I can't use my Gizmo number for my end of the call, but calls work if using cell phone or land line to handle my end of the call.

For the calls that fail it never rings my Gizmo number, maybe it's trying to dial my 1747xxxxxxx on the PSTN. If that's what's happening I'd never hear the phone ring because that would be someone's PSTN line, not my number.