If you work in a building that acts like a giant cell-blocking cage, enabling WiFi calling can help you make and receive phone calls when in areas where Verizon/ATT/Sprint can't touch. IMPORTANT: IF YOUR PHONE IS ON iOS 14 or greater, there are additional steps you need to take that can be found here.
1) Open your Settings app on your iPhone
2) Tap on Cellular --> WiFi Calling
3) Toggler WiFi Calling on this iPhone to ON
4) If you have an iPad or other device that's logged in to your iCloud account, and you'd like to make it capable of WiFi calling as well, you can toggle on "Add WiFi Calling for Other Devices" to on, too, but make sure it's a device you don't leave with students or laying out where anyone can access it.