BT Mobile has announced that it will now support data calling over both Wi-Fi and 4G connections. Data calling can mean audio calls even when a network connection isn't available but Wi-Fi is up and running – ideal for rural black spots.
For current BT Mobile customers their service is active now and can easily be activated in the phone's settings.
This service offers super high quality and clear calls for audio. The catch? This only works on handsets that support the service.
Initially, the roll out is mainly for iPhones. Wi-Fi Calling works on Apple iPhone 5S or newer and for 4G Calling you'll need an iPhone 6 or newer. The 4G calls will then cost you from your data plan.
In terms of other manufacturers, only the Samsung Galaxy S9 and S9 Plus bought from BT Mobile will be compatible. BT says that it will be announcing more compatible phones in the future.
Source: https://www.techradar.com/news/wi-fi-and-4g-calls-just-arrived-on-bt-mobile