WiFi-calling is designed to only kick-in when your mobile signal is either minimal or zero. It's also reliant on your home WiFi being of a suitable speed & latency.
Probably the best test you can try is to switch flight mode on (to force zero mobile coverage) and then enable WiFi individually and by itself. That should prove the service working one way or the other.
If it fails to kick in then, I'd suggest restarting your phone and router, and also make sure you're able to use other phone functions over the same WiFi network.
