Default [Q] Is htc one support HD voice?

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"??