Just wifi calling doesn t work???

There are two aspects to it, know it here

1. WiFi Calling --> Make a call with Wi-Fi Calling - Apple Support
2. WiFi Calling on other devices --> If your carrier supports Wi-Fi Calling on iCloud-connected devices, you can also make and receive Wi-Fi calls on other devices. 

WiFI Calling is Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency who issued the SIM / eSIM to you.

Get help

1. Go to Settings > Phone > Wi-Fi Calling and make sure that Wi-Fi Calling is on.
2. Restart your iPhone.
3. Connect to a different Wi-Fi network. Not all Wi-Fi networks work with Wi-Fi Calling.
4. Turn Wi-Fi Calling off and then on again.
5. Go to Settings > General > Reset and tap Reset Network Settings.

Make a call with Wi-Fi Calling - Apple Support

Your cellular provider needs to offer it. I was on O2 and had iwifi calling., then moved to GiffGaff on the O2 network and lost it.