Access Cellular Settings
Open the 'Settings' app and find 'Cellular' or 'Mobile Data' (names might vary).
Enable LTE and VoLTE
Inside Cellular settings, tap 'Cellular Data Options' or 'Mobile Data Options'.
Turn on Voice & Data
Look for 'Enable LTE' or 'Voice & Data', tap it.
Select VoLTE
Among LTE options, pick 'Voice & Data' or 'Enable LTE' to enable VoLTE.
Confirm Activation
If asked, tap 'Enable' or 'OK' to confirm.
Restart Your iPhone
After making changes, restart your iPhone for the settings to apply.