In order to turn this option on you will need to do the following (video demonstration also shown below):
- Swipe down from the top of the screen
- Swipe down again
- You should now see a lot of buttons including things like bluetooth, do not disturb etc
- Swipe left until you see Wifi calling
- Tap on Wifi calling
Now whenever you are connected to wifi your calls will go through that rather than the phone signal