To use N900 like a phone, without cellphone subscription . All you need is
1) Wifi
2) google voice account
3) gizmo5 account.

To receive incoming call:
6 blue squares>Settings>VoIP and IM acconts>New>
Sip
User name : gizmo5user@proxy01.sipphone.com
Password: gizmo5user password
Enabled check
Advanced settings (may skip if no dial out?)
User name gizmo5 user
Outbound proxy proxy01.sipphone.com
Port 5060
Done

New>
Google talk
User name gmail account user
Password gmail account password
Screen name anything
Save

Now you can receive incoming call thru your googlevoice number, when dial the googlevoice number your N900 rings, even when an earplug is in. Dont do it when in a wedding ceremony, your N900 will ring , loudly
More info to set up Gizmo5 and googlevoice (Grandcentral) here
Voice quality: Super!

To dial out:
1) Dialcentral, http://forums.internettablettalk.com...522&styleid=12, thread #887, #889 or
2) use your googlevoice dialout and set the phone to ring your gizmo5account

To dial out:
go to your googlevoice sign in, and enter the dial out number, your N900 should ring.
I have to use a desktop to do the googlevoice call. And it worked.

Voice quality: Super!

I know, this is pretty tricky and to people already has a cellphone subscription, it is not much. For me, this is another step to beefy up my N900, so that I can leave my N8x0 at home. Currently I am still carrying both, as I need the N8x0 to receive incoming call......... not any more, I can now leave my N8x0 at home, starting tomorrow

Enjoy,

bun