VoLTE is always enabled per operator and both sides need to make changes to make it work. Unfortunately, this is not something which we can simply turn on. The more ASUS users asking their operator about VoLTE, the faster we will be able to get their help.
It would be really nice if it was able to be enabled, even with the warning. I am now having to change mobile network when my contract ends as I get very poor signal at home and wifi calling was the only way I could actually hear people on calls! I'm worried though as I think that all the networks have the same issue at my home address, it seems to be a bit of a blackspot so without wifi calling I might have to get a different phone 😕