Since having this V10 (obtained it when it first became available), I found I cannot use VoLTE nor Wi-Fi calling. Two different issues for them, but neither work reliably.
For VoLTE, the phone will consistently drop any call every few minutes. Call initiation also takes an inordinately long time. And some calls, I've discovered, simply don't come through. I'll get a voicemail notification, but never got the call that initiated it. If I turn off VoLTE, the problems disappear, calls are steady and reliable. At my home location (where I am most of the time), I get full bars of LTE signal strength, I'm in the heart of Atlanta. T-Mobile tech support says I'm being served by 3 different towers, and likely my phone is constantly switching from one to another and that could be causing the calls to drop. Obviously, that isn't acceptable. Obviously, the technology needs improvement.
For the Wi-Fi calling, this phone seems to have a problem maintaining a Wi-Fi connection. Whether at home or out, the Wi-Fi will connect and be connected for a period, but then just drop. I even have a T-Mobile AC-1900 WiFi Cellspot Router, but that hasn't helped. The phone just drops the wifi signal at random intervals. It also does this at Starbucks (using Google WiFi) or other locations. My other devices (a Nexus 7 tablet, a Nook tablet, a WDTV box) don't drop the signal so I know it isn't my router or internet connection.
Hopefully, the new update to Android 6.0.1 will help? We'll see.