Wi-Fi calling not working

Wi-Fi calling works in addition to the cellular network, not instead of it. The phone will use whichever signal is stronger, and it will switch seamlessly between the cellular network and Wi-Fi if you move into or out of Wi-Fi range.

If you only want to use Wi-Fi calling and not the cellular voice network put the phone in Airplane mode, then turn on Wi-Fi.

Mine won’t enable

Your carrier must “provision” Wi-Fi calling on your account before you can use it. You need to call them and ask them to enable it.