Well, the app is well designed, intuitive, a breeze to use and above all, it works swimmingly on wifi. I tested calls on two different numbers and call quality was superb on both.
The bad news however is that on 3G, it is not currently an option. When you attempt to make a call, a message pops up saying:
“calls over 3G networks are currently not supported due to restrictions that may be placed on your data plan.”
The app is available for free and you can give it a try yourself here (appstore link)