I have an S7 Edge from O2 on an O2 contract. Sometimes the Wi-Fi calling switch in settings just disappears.
Somebody posted a fix that can work intermittently..Basically enable flight mode, then disable it, wait one minute then it reappears....sometimes... This to me seems very flaky and I have my thoughts on what their issue is and why this happens.
This also affects those who have the icon one minute and it's gone the next. Just use the flight mode get out. It will still keep on happening though until O2 fix their parameters. Seems like a simple resolution on the surface.