From the Home screen, navigate: Apps > Settings > Network & Internet > Advanced Calling.
If unavailable, navigate: Apps icon > Settings > More > Advanced Calling.
For Samsung devices on Android 8.x or later, swipe up or down from the center of the display to access the apps screen, navigate: Settings > Connections > Advanced Calling > then tap Allow from the 'Advanced Calling' prompt to complete activation.
Tap Advanced Calling to turn on or off.
To continue, review the disclaimer then tap Activate.
Review the added and removed add-ons then tap Continue.
From the 'Congratulations! you now have Advanced Calling' section, tap OK.
Restart the device in 5 minutes to complete activation. Check for incompatible add-ons that must be removed in order for HD Voice to be activated.
The voice portion of the call is billed per minute of use and the video portion is billed as data. For additional info, see these HD Voice FAQs.