Hello alex_whatever2,
Be sure to check that your current mobile carrier supports the Wi-Fi Calling feature. This page can help out with noting which features are supported by a carrier: Wireless carrier support and features for iPhone in the United States and Canada - Apple Support
If applicable be sure to select your region at the topic of the page.
This page will also provide additional help, if the carrier supports Wi-Fi Calling: Make a call with Wi-Fi Calling - Apple Support
If you can't turn on or use Wi-Fi Calling, make sure that your carrier offers Wi-Fi Calling and that your devices have the latest software. Then, follow these steps if you can't turn on Wi-Fi Calling or make a Wi-Fi call. Wait two minutes before you go from one step to the next.
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 > Transfer or Reset iPhone > Reset, and tap Reset Network Settings.
Cheers.