Wi-Fi calling allows your iPhone to place and receive phone calls and text messages over a Wi-Fi network. If you have a weak cellular signal but a solid Wi-Fi signal, your iPhone will automatically switch over and route calls and texts via Wi-Fi.
Apple added support for Wi-Fi calling to the iPhone with iOS 8, and it’s now supported on many carriers. In the US, AT&T, MetroPCS, Sprint, T-Mobile, Verizon, and Vodafone support it. You can only use this if your cellular carrier supports it.