T-Mobile is currently pushing out a firmware update for the Nokia Lumia 521 which brings a Windows Phone first to the handset- voice calling over WIFI.
The software will update your handset to firmware 1030.6409.1316.0016 but will unfortunately require a hard reset of your device, meaning your game progress and unbacked up data will be lost.
WIFI calling is ideal for extending the range of T-Mobile’s cellular network into bad reception areas such as basements or hilly areas, so it generally considered a very desirable feature.
To get the update:
Read more about the process at T-Mobile here.