If you can't turn on or use Wi-Fi Calling, make sure that your carrier offers Wi-Fi Calling and that your devices have the latest software. Then, follow these steps if you can't turn on Wi-Fi Calling or make a Wi-Fi call. Wait two minutes before you go from one step to the next.
Go to Settings > Phone > Wi-Fi Calling and make sure that Wi-Fi Calling is on.
Restart your iPhone.
Connect to a different Wi-Fi network. Not all Wi-Fi networks work with Wi-Fi Calling.
Turn Wi-Fi Calling off and then on again.
Go to Settings > General > Reset and tap Reset Network Settings.
FaceTime isn't available in all countries or regions.
In addition to the above poster's suggestions; also ensure that your carrier has given you the correct sim. Like for me, Straight Talk on the AT&T Network does support WiFi calling, but Straight Talk sent me the wrong sim, so the option for enabling Wifi Calling is not present in the settings of my 6s.