I don't know if I'm actually reaching T-mobile this way or just the community. There doesn't seem to actually be any way to reach T-mobile except by calling, which is hard to do when phone calls don't work.
I have full LTE signal on an unlocked LG G5 (RS988). When I initially turn the phone on I can make and receive phone calls for a while (maybe half an hour), and then it stops working. Incoming calls go straight to voice mail. Based on other comments it seems to be because T-mobile requires VoLTE, which I believe is not supported by my phone.
T-mobile, if anybody at the company is actually reading this, please fix your network, or if you believe it's a phone configuration issue, contact me through email and suggest resolutions.