What is WiFi Calling?
With Wi-Fi Calling, you can place a phone call if you have a Wi-Fi connection in an area with little or no cellular coverage. Instead of using your mobile carrier's network connection, you can make voice calls via a Wi-Fi network. That could mean using a Wi-Fi connection you have set up at home, or whatever Wi-Fi hotspot you happen to be on when you're out and about, such as at a cafe or library. In most ways, it's like any other phone call, and you still use regular phone numbers.
All four major US carriers (T-Mobile, Sprint, AT&T and Verizon) provide built-in Wi-Fi calling. See the carrier-specific links below to find out how to enable WiFi Calling for your mobile carrier.