I actually bought GrooveIP to use money I had in a Google account to call Israel from a Tablet.
Here's the thing- calling over 3G is VERY iffy. The speed is there, but the latency is NOT. Remember, VOIP does not actually require much speed... heck, a 2G connection has more than enough bandwidth for a simple voice call (somewhere around 64k).
The problem is latency.
Cellular data is very high latency, which means the time between a request and response is very laggy. That means that while the connection is more than fast enough to carry voice, the voice will either echo or be extremely delayed. There are tons of VOIP services that are optimized for mobile that try to get around the latency problem... Fring and the newer Skype mobile, for example, support high-latency codecs for mobile connections if needed (at the cost of sound quality).
But this app is essentially a hack. It is connecting and using the codec for desktop calling, which is NOT optimized for mobile.
End result? I made a call over a solid EVDO connection the day I got it to test. The person on the other end was crystal clear, but our conversations were about 3-4 seconds off from each other. Its hard to describe how annoying it is to have a conversation like that unless you've tried.
Over WiFi, however, GrooveIP is excellent (and why I bought it- turning my tablet into a giant google voice enabled phone is brilliant!). If you intend to be at home or someplace with WiFi most of the time, this can work, however the constant polling to keep the connection alive will likely prematurely drain your battery (I did this with Skype back when they allowed free phone calls, and it ate my battery in a matter of hours). Still quite useful and an excellent proof of concept. Just don't expect this to work as your described above.
