Hi, it's done through your network provider as it's them who need to activate it on your account.
See below for more tips
Initially Wifi needs to be activated on your mobile account. So contact your mobile network provider and check.
To check your device for WiFi calling setting, try these
1. Pull down from top of your screen into your quick access panel buttons and tap the WiFi calling button to turn it on. Long press on the button for the calling preference options.
2. Go to the phone app, tap on 3 dots, tap settings, you should see the WiFi calling option (if CORRECTLY enabled by your network) Toggle it on.
3. Settings> Connections. Toggle on WiFi calling. Tap on it for Calling Preference options.
Also see this link
https://www.samsung.com/uk/support/mobile-devices/how-to-activate-wifi-calling-on-a-galaxy-smartphon...Note: If you're on the Vodafone network then option 2 may be the only solution.