The Nokia 770 is getting better! The Tablet OS 2006 software release fixed the e-mail client, improved performance, stability and GUI and added Jabber IM and Google Talk voice. Pretty sweet in itself.
Just now, the Gizmo project has also released voip software for the 770. This allows you to actually call other telephones using your Nokia 
They say: “Now available for the Nokia 770 Internet Tablet, Gizmo Project offers crystal-clear WiFi calling with just a few taps of the stylus. Install it today to enjoy the freedom and low calling rates that Gizmo Project provides!”
I can’t seem to call mobile numbers. Landlines are okay though (yes, I first purchased credit 
)
A pity though that other SIP accounts are not configurable.
I’m in the UK and have managed to call mobile numbers but there is a very long delay. First it ‘calls’ Gizmo and then appears to pick up the call, you then have to wait say 10 seconds before you should hear it calling the other end. So far I get about a 50% success rate. I figure it’s the transit across to the US, and then back to the UK that causes the delay. This is pretty much the same on the Mac when I used Gizmo on it.
I have just retried and it worked
btw, check http://www.gizmoproject.com/learnmore-allcallsfree.html
not really free (the ppl you call have to be gizmo users too) but getting there