I forget the details of how it works, but it worked for me using a BT App on my O2 mobile, with O2 broadband on a BT Retail landline. Instead of (for example) 0800 calls from my O2 phone/O2 call plan costing me, in the days 0800 wasn't free on mobiles, I used the BT App and that call was "charged" (= free) against my BT landline.
That's something completely different of course, and works if you have a BT landline - even works if you are overseas.
The WiFi calling feature is something built in to all iPhones (since the 5) and some Androids and works on EE and Vodafone today (Three coming soon) without adding an app. Vodafone only with 6S or later.
Its a good service if you have reliable low latency WiFi that isn't being maxed out by others, e.g. hotels and shopping centres. I tend to keep it switched off and only turn on if I'm somewhere with "No service" displayed which is a lot less on EE than it was on Vodafone.
plusnet unlimited fibre 80/20 since 2 Jun 14 - Sync as of 7th Aug 16: 55,355/10,291 kbps with G.INP
17 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM