What phones does it work with?
EE previously confirmed to TechRadar that it was working with HTC before we heard of the One M9 getting support - and there was a suggestion the One M8 may see it at some point too.
Your handset has to come from EE, as it needs the network's software build installed to enable the Wi-Fi calling feature. If you buy yourself a SIM-free S6 from elsewhere for example, sticking an EE SIM card into it won't give you this functionality.
Vodafone's Wi-Fi Calling service has not launched yet and we don't have official word on the compatible handsets just yet.
Meanwhile Three's inTouch app is available on Google Play and Apple's App store, and O2's TuGo offering can be downloaded on iPhones running iOS 5 and above, Android 2.3 and above, Windows Phone 8 and desktop web browsers.
How do I enable Wi-Fi calling?
We can only speak for EE at the moment, so once you receive your Wi-Fi calling enabled handset from the network you will need to text "wifi calling" to 150 to set it up.
You will need to able to flick the switch on the Wi-Fi calling option in the settings menu of the phone app. On the Galaxy S6 and S6 Edge (and probably the S5 too once the update arrives) just open the phone app, hit 'more' in the top right corner, 'settings' and then scroll to the bottom of the list to turn on.
Your handset will then automatically switch to Wi-Fi calling when it connects to a saved network.
What does it cost?
Nothing! Well, nothing additional on top of your phone contract and home internet bill. The same goes for O2's TU Go and Three's InTouch, which are free apps customers can download and use.
It's worth noting if you've not got unlimited calls or texts, calls and messages made over Wi-Fi on EE and Vodafone will come out of your monthly allowance.