Wi-Fi Calling lets you make and receive calls and send messages over a Wi-Fi connection when your mobile network signal isn't available. For this feature to work, your phone must be connected to a Wi-Fi network, have an active SIM card inserted, and Wi-Fi calling must be enabled by your network provider.
You don’t need any apps for WiFi Calling. Unlike FaceTime, Skype, Whatsapp, Viper, etc., you don’t need any special apps to make and take calls or send & receive messages. WiFi calls are like regular calls—you can just use your phone’s built-in dial pad or messaging app.
