I have always had issues not be able to call and over wifi for the unlocked phone with att as my carrier. I have tried contacting both Samsung and att multiple times with no luck, each blaming the other for the issue.
Finally today the option popped in quick settings panel out of no where.
Looks like they were able to get there shit together at last hopefully this is true for all the other unlocked models
Did you update the phone recently. If so then we just have to wait. It's been reported since note 8 age as far as I'm aware and it is finally fixed for note 20 ultra. It's just a matter of time till the update comes to other models. Hope that would be soon
The wifi calling feature came along with the security update on 18th as stated by Ksal. It you haven't updated the phone yet. Just check if there are any pending updates available for you. That would be under settings - - > software updates - - > Download and install.
Also check the last update date in the same settings section
