As reported earlier T-Mobile is now rolling out a new OS 10.1 update to users that includes Wi-Fi calling as well as a number of improvements and bug fixes. The update comes in at OS 10.1.0.4780 and is around 150MB so it shouldn't take long for most users to update. If you're on T-Mobile and haven't yet received the update you should be seeing it soon. You can always check manually by going to Settings > Software Updates.
After installing be sure to give your device a bit to kick in at it sometimes takes a while for the OS to get itself sorted.
Swing by the forums and let us know how things are working for you!