How can I enable calling over Wifi for a T-Mobile based Moto X?
The Moto X does not support T-Mobile's WiFi calling. Customer support reps have stated that this is because it is not a carrier branded device, meaning they have not modified the system ROM to add this feature:


  The Moto X does work on our network but because it is not a T-Mobile branded device we do not support WiFi Calling on it.


You'll either need to set up some other VoIP service or get a different phone that supports this.