I am a Lebara customer, which means I use the Vodafone network too. I used to be an EE customer and their service could be configured to prefer Wi-Fi calling over the mobile network for Wi-Fi Calling. I never had a single problem with EE's Wi-Fi Calling service. However, EE’s punitive roaming charges made be consider an alternative provider and Lebara are great value for money.
Lebara’s network operator (Vodafone), in its extreme incompetence, does not allow you to specify this preference. The marginal nature of their service therefore means calls cannot be made and received reliably when you have a poor mobile signal but good Wi-Fi. It gets worse. Even when I'm on Flight Mode and with Wi-Fi Calling active, calls are still not getting sent to my handset. This is an example of a mobile network operator that cannot competently operate a mobile network.
To lighten your load, if you're still a Vodafone customer, there is something you can do on Samsung handsets for Wi-Fi calling to be on all the time when Wi-Fi is active. Go to 'Modes and Routines'. Create a routine that activates Flight Mode automatically when you connect to your Wi-Fi network(s). I'm not sure whether you need another routine to actively deactivate Flight Mode when you disconnect but I have this and it works reliably, unlike Vodafone's Wi-Fi Calling service.
