EE, the amalgamation of Orange and T-Mobile in UK, and the largest carrier in the region, has announced that it will start rolling out WIFI calling on the 10th April.
At launch the feature will only be supported by 3 handsets, one being the Lumia 640, and the other two being versions of the Samsung Galaxy S6. The feature uses support built into the operating systems, and will not need any additional software.
It will also eventually come to the iPhone 5S and 6, but only with version 8.3 of the operating system.
Calls will work exactly as when connected to your carrier network, using your existing dialer, and will only require one to be connected to WIFI, and will use minutes from your plan as usual. Calls should be able to transition from WIFI to LTE without interruption.