When I was in the UK a couple years ago I could make calls back to USA using WiFi, because the wireless service was incompatible with my phone.
When I was in the Dominican Republic last year, even though I used GrooveIP and Google Voice, I came home to a $400+ wireless roaming bill, I think because the wireless service there IS compatible. So how do I force my phone to make calls using WiFi, even when wireless is available?