This is an issue that should have been resolved by now. My unlocked Pixel 3 had wifi calling on AT&T because Google worked with AT&T to get it working. The same cannot be said about Samsung which is a shame. The US Unlocked Samsung phones should have wifi calling working on ALL carriers. Yes I could go and buy the AT&T version but then I get all the crapware in addition which I don't really want.
@slane on AT&T, before wifi calling works VoLTE has to be working first this is because when wifi-calling is out of range the call will hand over to VoLTE with out disconnecting but it will not hand over to 3G calling.