I have a dual sim phone that I bought a few years ago from a local retailer that sells a lot of imported models. Used it as is for the last few years, but recently flashed the NZC (2degrees) firmware onto it to see if I could get wifi calling to work - the NZ version of my phone is supported officially, so I figured that was a good start.
Have lost the ability to use the second SIM slot of course, but was pleasantly surprised when I was able to enable wifi calling and it seems to work pretty well. Not being able to "roam" off wifi onto the mobile network during a call (e.g. if you're leaving the house) is a shortcoming though.
Has anyone else tried this? I know the 2degrees website says overseas phones aren't compatible, but it seems this is maybe more a throwaway line of not wanting to support endless combinations of device configurations, rather than a technical limitation.