From the Home screen, navigate: Apps icon > Settings > Advanced Calling.
If unavailable, navigate: Apps icon > Settings > More > Advanced Calling.
For Android 8.x or Android 9.x OS, navigate: Apps > Settings > Network & Internet > Mobile network > Advanced > Advanced Calling.
For Samsung devices on Android 8.x, swipe up or down from the center of the display to access the apps screen then navigate: Settings > Connections > Advanced Calling > then tap Allow from the 'Advanced Calling' prompt to complete activation.
For LG devices on Android 8.x, navigate: Settings > Network & internet > Advanced Calling.
Tap Activate Advanced Calling.
To continue, review the disclaimer then tap Activate.
Review the added add-ons and the removed add-ons (if any) 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.