I’m not sure about SMARTY, but I’ve had issues with 3G reception where I live. So a year ago I looked into 4G calling. I tried third-party provider Virgin mobile, but 4G calling/VoLTE didn’t work on my FP3. I asked on the beta forums a year ago and got the following response
which is fair enough. I understand that it’s not trivial to add in providers.
So I had to go with a main provider (in my case EE) which is supported and works very well for both 4G and Wi-Fi calling on my FP3. So I would guess that Three would be fine too. I’m not aware of a list of supported providers. Might be worth checking with Fairphone Support.
Hope that’s useful.