There are few things that you can check on your phone if the Wi-Fi Calling feature isn't available.
First, make sure you are connected to a Wi-Fi network. If the network signal is weak or unstable, the Wi-Fi Calling feature may not work. Open the Quick Panel and check the Wi-Fi icon to make sure you are connected to a Wi-Fi network.
In addition, to use Wi-Fi Calling, you need to insert an active SIM card from a carrier that supports this feature. Check with your carrier if your SIM card is supported. If that doesn't resolve the issue or if access to the feature isn't allowed, please contact your carrier for further assistance.