And VoLTE is not supported on those handsets officially so I’m not going to dish out money just to try(I’m with Kogan under VFNZ)
You may be better off sticking with Kogan if you don't have an official device.
I have a PH-1, which was never sold in NZ but has essentially pure Android, including all of the default VoLTE / WIFI calling Options. I noticed earlier in the year when I was doing a test, VoLTE worked fine on Kogan, but doesn't on any other main networks tested.
I moved to Kogan due to their 2 for 1 offer last week and VoLTE has still been working perfectly. Just now, I received my first incoming WiFi call on Kogan on my privately imported PH-1 and it worked perfectly. Hopefully posting here won't make anyone decide to block this from working, but, for me at least, Kogan, through Vodafone seems to enable more features rather than Vodafone directly who lock down / disable if not on a whitelist.
I understand the need to test, but, once my 1 year is up, moving back will have to be carefully considered if it results in removed functionality.