O2 is finally ready to let people make calls through 4G and Wi-Fi connections, as it launches its own take on the coverage-expanding trick that the rest of the UK's mobile networks use.
Seeing as it's a posh new thing it's only launching on the posh new phones at first, with iPhone 7 and 7 Plus users on any of the network's PAYG or SIM-only deals now able to use it once the required software update has been received on the phone and the carrier bundle has been updated or changed or whatever needs to be done there.
After that, starting of calls away from the mobile network via Wi-Fi and seamless transfers to mobile when you pick up a signal should be able to happen, plus it can now use the 4G network (in London, Leeds and Slough at first) for voice calls, without needing the Tu Go app that previously routed O2's network happenings to a Wi-Fi connected users.
Source: http://www.gizmodo.co.uk/2017/03/o2-joins-wi-fi-and-4g-calling-party/