T-Mobile users in the USA can now grab the latest BlackBerry 10.1 update through the software update menu.
The update brings some new features and bug fixes as well as wifi calling. The update is around 150MB so should be quite quick to download and install. If you haven't received your notification for the update, you can check now by opening Settings, and hitting the Software Updates menu.
Let us know how you're getting on with the update and the wifi calling!