The option appears in my Nokia 6 as well, but doesn't do anything yet. In theory it should work, HMD make a bit of a deal about the phone being VoLTE and VoWiFi ready. We'll see.
Most handset I will imagine will have their built in firmware updated to support a native Wifi & 4G calling, now that BT have joined the those offering the service the firmware will include the BT update which is why its now shoing prior to launch.
It then requires an OTA update and some back end magic at BT's side to enable the account once the service is launched. Maybe the easter bunny will trade in my mini eggs for wifi calling - heres hoping
It seems that the 11.3 (beta 3) did bring the patch for wifi calling to the lastest Apple handsets. You are able to toggle but it wont work until the network enables your sim card/ Account.
You can tell on any network when you are on a wifi call as it will display it next to the Network ID at the top of the screen. And in settings in iOS under Carrier.
And don't forget 4G calling (VoLTE) should come at the same time!
It will be like waiting for a bus, nothing....then two at once!! 4G & Wifi Calling will make alot of people very happy!!!!