Ok, for google voice,

You are going to get a new T-mobile number ANYWAY.

Instead of giving people that number, give them your google voice number.

When you have cell coverage, Google Voice will ring your phone, and it will work like normal. People will see your google voice number when you text them or call them (using google voice of course). The dialer app for the N810 should take care of this.

When you have no cell coverage, it can route your calls via SIP and WiFi. The callers see no difference either way.

Google voice has been really helpful to me. I have zero cell coverage in my dorm, so i use it to route to my landline and n810. And the best part is that i can pick up the call on the landline, switch to my n810, leave the room, switch to my cellphone, and leave the campus, all in the same call. Sweet...