Windows Phone 8.1 includes native support for WIFI calling, which means on the right carrier you can make and receive phone calls over WiFi, and also send and receive SMS messages and MMS messages.
The feature was previously available via an app on T-Mobile, but now that the service is integrated users report it is much more reliable.
When on WIFI a phone icon overlays the WIFI icon, indicating the option is now available. The phone tile on the home screen also switches from saying T-Mobile to saying WIFI.
Users have the option of preferring WIFI over cellular or vice versa, or switch to WIFI only, which can be useful in a roaming situation.
The option is now available on the Lumia 640 and other Update 2 handsets when they arrive on the carrier.
Have our users tried out this feature? Let us know below.