If your mobile contract includes the option to perform so-called "Wi-Fi Calling", calls via Wi-Fi, then you must also do activate that feature in the system settings of the Samsung Galaxy S8. Only then is it possible to use the option from your mobile phone contract.
We tell you here where to find this attitude.
Instructions to enable Wi-Fi calls on the Samsung Galaxy S8:
1. Open the phone app on your smartphone
2. Tap the three-dot icon in the upper right corner - A pop-up menu will appear
3. In this select "Settings"
4. Now scroll down and enable the "Wi-Fi Calls" slider to make the necessary settings on the Samsung Galaxy S8.
You should now be able to use Wi-Fi Calling with the Samsung Galaxy S8. Under circumstances, your mobile service provider may need to unlock additional settings.