T-Mobile has been pushing some new network technologies lately, like the Advanced Messaging platform announced a few months back. Now it's moving on to video calling by adding native support to its network for select devices. It will require a software update to use, but the experience of placing a video call should be somewhat less annoying.
T-Mobile's video calling system will be smart enough to adjust to your network conditions. It will be accessible via a button in the dialer app when you're calling someone who also has a supported phone. Video calling works over LTE or WiFi, and the call will automatically step down to voice-only if you move off a sufficiently fast network. If you're going to use video calling, this seems like a good way to do it. Although, it doesn't sound like this will work across networks, for example with Verizon's Advanced Calling video. Carrier interoperability is pretty messed up these days.
The data used by a video call on LTE will count against your monthly totals, which I'm actually a little surprised to hear. It would have been easy for T-Mobile to exempt its video calling from those limits, but that wouldn't have been very network neutral of them. The video calling update is available today on the Note 5 and S6 Edge+. The regular Galaxy S6 and S6 Edge will get the update next week. Everyone else will have to wait and see if their device gets support.