I noticed that a new icon appeared on my phone last week, EE Wifi calling but I didn't give it much thought until today.
I looked into it and it appears that while I'm connected to a wifi source, my calls are sent through the router rather than the closest cell tower. With the thickness of my walls, this is an obvious advantage for me as I don't have to go outside in all weathers to chat with my friends.
It's not always enabled on Android phones so you may need to muck around in the settings, I've got a Nokia and they're pretty good for upgrading new features like this but it's best to check.