Yes, I used WiFi calling (and SMS) on 2Degrees during a recent 4 week trip thru the USA and Central America - worked perfectly everytime I needed it to.  Headset was an Oppo A74 5G.

TommySharp:

 

Has anyone had luck with getting wifi calling to work with the OnePlus 8T or any other phone that is not Apple or Samsung?

 

Oneplus 8T is not a supported phone

https://www.2degrees.nz/help/mobile-help/calling-features/phones-compatible-with-volte-and-wifi-calling

Thanks, so pretty much just the phones that 2Degrees sells then :-)

TommySharp:

 

Thanks, so pretty much just the phones that 2Degrees sells then :-)

 

some phones allow the carrier check to be disabled.

e.g. My Redmi Note 11 out of the box doesn't support 2D VoWiFi.
However, I am able to disable the carrier check and then it works just fine.