All UMA is doing is using the wifi as if it were a cell tower. No extra charge if you made a call over it or text message. Same as if you were doing the same thing over EDGE. So if the call would use minutes on the cell network it would use minutes on UMA.
It's like the difference between say GSM and EDGE. Just a different type of equipment, but same charge for the call.
Oh yeah, on the wifi, there's a way to use it without a data plan. There's a theard in the wifi forum on it. 8320 without a data plan -- What you can / can't do