Using the truphone app on ipad, you can make and receive voice calls, even with the wifi version, which is what I have. The call rates are reasonably cheap, and voice quality is excellent. However, it gets better than this. You can use Google voice + Gizmo + Truphone to make free phone calls to anyone in the US, right from the iPad. Of course you need a wifi connection, but isn't this cool?
Here is how to do it
1. Sign for a truphone account, and install their iPad app.
2. Sign up for gizmo and set it up to forward it to truphone sip account (email@example.com). The glitch here is that since google's acquisition of gizmo, new signups have been disabled. I was lucky to have an old account lying around.
3. Signup/ signin to google voice and set it up to forward it to gizmo number
4. Go to google voice page in browser. From the bottom link, select the desktop version, as there is an issue with the mobile version which opens up automatically.
4. Make that call, choosing to let google voice connect you using the gizmo number.
That was how you make outgoing calls. Here's how to make incoming calls.
Now while truphone app is good enough, it is still not supporting push, which means you get your calls from google voice only if truphone is running in the foreground. If you have jail broken your phone using spirit, you can install backgrounder and run the truphone app in the background, which would enable you to get the calls all the time, as long as your wifi connection Is up ( or if you are on 3G already)