I've got an NZ Oppo AX5 (bought from the Warehouse). As far as I can tell it should work with wi-fi calling.
With a Boost AU SIM in it, I see the wifi calling icon on whenever I'm at home.
With a 2degrees SIM in it, I've never seen the wifi calling icon in.
The phone also has a Skinny SIM in it (dual SIM phone). 2d/voda reception here (small town on SH8) is highly variable.
Edit: obviously I forgot to ask my question: should I expect 2d wifi calling to work on this phone, on a boring home network (HG659, Tenda Nova mesh Wifi in bridge mode) on which Boost AU wifi calling works?