Cawtau is right!
At this time, the LG V20 is not compatible with Wifi Calling.
In order to use WIfi calling, you must have an eligible device purchased from Fido.
Eligible devices are as follows:
LG G4 (must be on Android 6.0 or above)
LG G5 (must be on Android 6.0.1 or above)
LG G6 (must be on Android 6.0 or above)
Samsung Galaxy S6 and Samsung Galaxy S6 edge (must be on Android 6.0.1 or above)
Samsung Galaxy S7 and Samsung Galaxy S7 edge (must be on Android 6.0.1 or above)
Samsung Galaxy S8 and Samsung Galaxy S8+
iPhone 5c, 5s, 6, 6 Plus, 6s, 6s Plus, SE, 7 or 7 Plus with iOS9 (or later) installed
Hope this helps!
Welcome to the community!
As far as I am aware, the service is limited to certain devices. Currently, the V20 is not listed as one of the available devices (see here). Additionally, is your device from Fido? Wifi-calling will not work using non-Fido android devices.
If the V20 is one of the devices which is compatible with Fido Wifi-calling and it isn't working, it's possible the OS update due out tomorrow might address that issue (see here). You should note that OS updates are rolled out in batches so your device may or may not receive that particular update tomorrow. However, you should receive the update in the near future.
Hope this helps