With Wi-Fi Calling, you can make and receive calls and messages over Wi-Fi, just as if you were on the network. This is particularly helpful in areas where the cellular signal is weak or nonexistent (basements, rural areas, etc.), but there is a strong and reliable Wi-Fi signal. Wi-Fi Calling also lets you to call the U.S. from anywhere in the world for free! And T-Mobile phones have Wi-Fi Calling built right in.