According to a video I just watched the Samsung Dart T499 which is one of the phones sold for this 30 dollar plan has built in VOIP calling. You can set the phone to make all calls over data by default. The phone is 120.00 plus tax and the service is 30.00 so you could walk in get a new phone and service and walk out for about 170.00.
The Dart is a very entry level Android. Do not expect anything great out of it. But for a phone that is just over a bill and will can be set on board to make data only calls. It would be worth a try. If it comes to the TMob stores here in St Louis I will be giving it a try.
And I just got done looking at wifi calling in the dart user manual and it says that using wifi calling does count against your min.
So another free voip service will be needed.