Hi @Stamatis1, let me see if I can help out here!
In order to be able to make calls with Wi-Fi Calling some conditions must be fulfilled:
- you have to use a phone that supports Wi-Fi Calling.
- your operator and subscription (incl. SIM card) must support Wi-Fi Calling for the device model you're using.
- depending on operator, your phone must sometimes be operator-branded and/or use operator-customised system software.
- your phone must be in range of a Wi-Fi network.
If all these conitions are fulfilled you should be able to enable Wifi calling
- Open the Phone app.
- Tap the menu button (three vertical dots) > Settings > Calling accounts > Tap the name of your service provider> Wi-Fi Calling. If Wi-Fi Calling isn't available, contact your network operator.
- Tap the switch beside Off to enable Wi-Fi Calling.
I hope that helped, let me know if you have any further questions! 
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