Last Beta, S9+.
WiFi calls and 4G calls, I know that this is an operator service, but this function periodically disappears. The operator said that on his part everything is OK and the service is activated. So apparently some of the Beta version bugs ..
i am also having wifi calling and VoLTE issues. wboth worked fine on oreo. now wificall doesnt work at all and VoLTE is flaky, sometimes i need to toggle airplane mode to allow me to make calls.
You are looking in the wrong place, this is for the app itself, to toggle it on and off go to phone, then settings and you should see it.