2 of the 3 Australia mobile carriers, Vodafone and Optus, geo-block WiFi calling and only allow it from Australia. You can use a VPN to overcome this, but you can't use an on-device VPN (at least on non-rooted Android) as Wifi calling bypasses the VPN. I have a separate Wifi network at home that routes over a VPN back to Australia, and can use that to make calls from Optus (haven't tried Vodafone).

At least initially they claimed they did this to prevent fraud/etc, although I suspect it has as much to do with forcing people to paying roaming rates as anything...

As I said above, I'm not aware of any US carriers that do this.