T-Mobile to soon offer Wi-Fi calling services for certain Android models2
Unlike the current UMA service, calls can not be handed off which means once you leave an area of coverage, the call you are on will be dropped. The service also will not be made available to all Android handsets. And if the G2 doesn't launch with Wi-Fi calling, don't jump from the highest floor in your building. It could be something that T-Mobile will add in a future software update.
T-Mobile G2 Specifications