They have to test the MBN files to work with Emergency numbers properly in each country before they release VoLTE for a phone.
That said, ASUS are completely silent on the issue, it's the same here in Australia and our 3G signal will be turned off here in a couple of years, meaning these phones won't be able to make calls. I'm pretty sure they just don't care, I don't see them ever responding to the numerous threads about this.
Fun fact, I actually CALLED Asus Australia's handset support regarding this issue and the Asian guy on the phone almost had no idea what I was talking about. He thought I was talking about VoIP initially and eventually told me it's a carrier issue and to speak to Telstra once he properly understood what I was talking about. Needless to say, it's not a carrier thing, Asus has to implement it.
See my extensive write-up here:
https://zentalk.asus.com/en/discussion/45081/volte-still-not-supported-in-australia-despite-taking-5...
