Doing some additional investigation on wireless calling ( assumption via WIFI ), not all carriers support it. ATT does via a separate device called a microcell. This allows calling over WIFI to the internet using any cell phone that has wifi capability. Newer phones support direct calling via wifi but limits are on those. your service or phone may not support the ability. Generally android and new IOS phones can.
T-mobile requires an app to be downloaded from their site to support the WIFI calling.
more info T-Mobile requirements for Wi-Fi Calling on BlackBerry 10 OS