WiFi Calling is an enhancement for T-Mobile phones that allows you to make calls over a WiFi network (your minutes are still used up). This function is great for those who have a weak cell signal in their house and still want to use their cell phone for calls. The other new feature is Mobile HotSpot support and one that I use with my HTC Flyer, Zune HD, and other devices with my current T-Mobile smartphones.
In addition, this latest update adds the following:
There are also some bug fixes, such as resolving audio issues and other freezes. If you own one of these devices on T-Mobile then you definitely want to get this update installed. I understand there are some significant improvements to BlackBerry Bridge on the PlayBook and have been thinking about another Bold 9900, but the price is still way too high for one.