Hi, s1391437164aa
Some carriers support a newer called called "Wi-Fi Calling" which eliminates the need for a cell network to place or receive calls. Having this feature turned on will indicate next to the cell signal bars when WiFi will be used in place of a cell network. For example, my Verizon iPhone supports Wi-Fi Calling. When the feature is turned off, I see "Verizon" in the upper left corner. When the feature is turned on, I see VZW Wi-Fi in the upper left corner instead.
To see if your carrier supports Wi-Fi Calling, go to Settings > Phone > Wi-Fi Calling or contact them directly
Hope this helps!