With Wi-Fi calls enabled, your Android phone uses the internet instead of cellular. As such, you can be in an underground cave and, as long as there’s a Wi-Fi signal, you can chat to whoever calls or will listen to you. Some remote rural places have Internet connectivity but no cellular signal.
Phone menus differ, so maybe it is easier to just search in the settings for Wi-Fi.
See https://www.tomsguide.com/how-to/how-to-make-wi-fi-calls-on-android