Even in cellular 4G hot zones like central London, mobile phone coverage in buildings can often be poor to non-existent, so WiFi Calling allows you to make and receive calls on your smart phone over your wireless network. The following mobile operators are already making use of this feature:
•O2 have a Smart Phone app called TuGo
•EE, as you have seen advertised by Kevin Bacon on the Tube, have enable smartphones to automatically use WiFi for calls.
•Three also have the ability to make calls over WiFi.