I've never liked relying on wifi voip for phone coverage. It's a nice idea, but a giant pain when you run into numerous network issues. Not to mention the fact that it's common to find public hotspots that don't allow voip.
That being said, why would you bother with the wifi-voip if minutes are unlimited? I'd just as soon disable wifi if I will be making a call and rely on the cell tower... Unless, of course, their specialized phone doesn't allow you to turn wifi off--which would majorly suck since your battery would die very quickly...