Default [Q] Is htc one support HD voice?

HI~
I'm using htc one 801s. My carrier support hd voice.

My phone checked "only for internet calls".

Q1. What's the internet calls? Is that mean WiFi or data(3g,4g)??

Q2. If I want to use hd voice, what do I check one of 3 options in attached picture.

Q3. What's the difference between "only for internet calls" and "for all calls when data network is available"??