With your phone connected to WiFi, select the 'Phone' icon, tap the 3 dots in the top right hand corner, select 'Settings', scroll down to WiFi Calling and switch on. Let us know how you get on.
@TP75: Just to add to what ChrisM said there, Wi-Fi calling is also network-dependent, so it's best to check with your network to see if it's available in your case (for example, some networks only have the feature on their branded phones).
I have been told by Vodafone that I have Wi-Fi calling. I have a Galaxy A5 (2017) which is listed as having it, but I cant find how to activate this. I have only found this but Wi-Fi calling is not there on my phone:
Please ensure Wi-Fi Calling is enabled in your My Vodafone app or in your My Vodafone account.
In My Vodafone
Go to Services and extras > Calling options > Switch Wi-Fi Calling On
I saw that you may have to register, but I don't know how to do this. Please help.