WiFi calling only works if you're on a reasonably good 4g signal. In call settings, at the very bottom you should see "enhanced call service" with options "voice over lte" and "wifi calling". Enable them.
Huh? lol
This is SO wrong. WiFi calling is for people who are getting a bad signal over the mobile network. If you had a good signal, why would you want to call over wifi? If you have a limit on your account for voice calls, wifi calling will still count against that. It's not VoIP.
Okay further to this, I tried H3G rom on htc and US unlocked ( i have S-off).
Dont find the option of enhanced call service. I believe sim has to play part in that, as when I put this sim in oneplus 5T it did not show enhanced call setting but when i put EE uk sim settings magically appeared
Now I am making calls on 4G so i take it its VOLTE its working on its own. dont find option of wifi calling in rom itself