WiFi calls require certain changes to the internal work of Android software (installed so that only an app is not sufficient to have wireless function). Phone details are: Model number SM-G930U baseband version G930UUE4APL2 Carrier is Verizon I have HD Voice but enabled I The Settings menu is no indication of WiFi calls. Gamestop Iphone 5s Unlocked there. Wi-Fi Calling is a nice little feature that actually does exactly what the name suggests. You can call over WiFi, which gives some battery life phone clearer phone calls. My Androids open source nature, it maybe a way to include the necessary internal OS items needed to install a T-Mobile WiFi calling -app (apk) to activate. I only use it for a day, but it seems that it automatically switches to Wi-Fi if the mobile network signal is weak and then switches back to your carrier as soon as the signal is strong again.