Wi-Fi Calling allows you to make and receive "phone" calls and send MMS and SMS messages over a Wi-Fi network. To do this you will need to have a compatible Wi-Fi Calling device, be connected to a supported Wi-Fi network and not have sufficient mobile network coverage to make a regular call. You must be connected to a Wi-Fi network and have the correct options turned on in your phone, VoLTE and Wi-Fi Calling. Then from Connections > Wi-Fi Calling and make sure its turned on. While using Wi-Fi calling there is a different icon at the top of the display.

Source: Bangkok Post November 07, 2017 21:45 UTC