I too have this issue, since upgrading to the latest software on my Samsung S7 I am now unable to use Wifi Calling. It is on none of the menus, I even wasted 2 hours of my life yesterday doing a factory reset but to no avail. Today I spent 1 hour 30 on the phone to both Three and Samsung - neither party claimed it was their fault and to call the other one. I'm so cross right now as the closest I got was the customer service guy at Three (when I escalated my call to a complaint) took my details and said he would call Samsung and call me back this afternoon
It all worked fine before this and I heavily rely on WiFi calling as the signal at both work and home is pretty rubbish!