I just recently updated my phone to Marshmallow and I can confirm that I finally have HD calling now. I'm not sure if it's on 100% of the time, but at least when I call another person who has a Moto X Pure Edition phone on T-Mobile, I get the HD icon under the person's name. I can also hear the difference. It is a bit clearer, but certainly not earth shatteringly clearer. Still, it's a nice improvement.
Now hopefully, we'll get wifi calling on T-Mobile soon...
Now hopefully, we'll get wifi calling on T-Mobile soon...