Most probably that VoLTE isn't enabled for your carrier. There's nothing you can do unfortunately. ASUS in your country needs to talk to the carrier and take it from there. It's a process that takes months but I recommend you to call your local ASUS and ask them
Even Rog phone 2 and 3 don't have VoLTE support in Thailand until now which is weird since max pro series has it I hope when Rog phone 5 announce will support VoLTE and hopefully for other phones via update in the future. Just like T-Mobile in the US
