If you pull down on the top of the screen then expand it further, is it in there? I'm guessing you've checked within SETTINGS-CONNECTIONS-2nd option should say WIFI Calling. If it's not in either location you can try a few things... Hold down volume button and power button until it restarts which possibly can clear the issue. Another option is do a factory reset but be sure all the info is backed up on a cloud. I'm not sure there's a carrier who doesn't offer Wi-Fi calling by now but if you're not in the United States I guess that could be the issue but even so, they cannot control what software is on the phone only if it'll'll function or not.
