A
Advanced Calling refers to a feature which enhances the sound quality of voice conversations on your phone. With the feature enabled, calls are in "HD" and sound like you're having a conversation with someone in the same room.
This feature is typically dependent on your carrier.
Yes but again, that feature is carrier dependent.
It may be that not all phones will show that a call is in HD. Which carrier do you have, and did you get the phone directly from them, or did you get it unlocked from somewhere else?
Advanced Calling is essentially VoLTE (Voice over LTE), which allows for the higher fidelity call quality, as well as video calling (if your carrier supports it).