Enable Volte for everyone its still disabled In Android Pie
This topic has been brought up a lot and it is happening for ROG Phone ll and there are quite a number of threads out there.
This will involve both ASUS and your local carrier/service provider.
ASUS will have to reach out to your carrier and your carrier has to agree to support the device. Once that is done the VoLTE will turn on by itself and it should be automatic. The reason why it is turned off is because most of our devices needs to IMS registration from our carrier. Without the carrier registration we will not be able to use VoLTE.
Hope this helps.