The Amaze 4G on T-Mobile isn't the only handset to gain Wi-Fi calling. Yesterday, T-Mobile released an update for the Samsung that adds the feature along with bugfixes to improve battery life and Caller ID. It's available now as an over-the-air update but if you can't wait there's always Samsung's Kies desktop software. The update also bumps the version number up to Android 2.3.5, but obviously we'd like to believe that 4.0 is waiting in the wings.
The Wi-Fi calling feature falls in line with T-Mobile's trend of going with the IP Multimedia System (IMS) protocol instead of UMA, a trend that looks likely to arrive on BlackBerry smartphones at some point.