The same time that they enable VoLTE and VoWIFI in Australia probably...never.
I was told by support that they would be testing VoLTE for the Australian version of the ROG PHONE 3 at the end of this year or Q1 2022 and we might get support for it by end of 2022. No idea if they are bothering with Switzerland though.
They also said they may not be adding the VoLTE functionality if they get it working, for the ROG PHONE 3. They may only add it to the ROG PHONE 5 or future 6.
I am switching to Samsung after my ROG phone 3 breaks or in a few years when the ROG phone 3 stops working in my country as 3g is being turned off in 2 years.