Wi-Fi Calling
Wi-Fi Calling for when there is little or no cellular coverage
Wi-Fi Calling can be used in campus buildings or other locations where there is Wi-Fi service but little or no cellular coverage.
Wi-Fi Calling provides cellular connectivity to smartphones over wireless networking, enabling you to make and receive calls and texts even though your device is not receiving a cellular signal. Once you enable Wi-Fi Calling, phones with that capability will switch to using it automatically when the cellular signal is low and the phone is connected to a Wi-Fi network.
Tell me more
Here is an article about Wi-Fi Calling from one of the IT news sites.
Everything you need to know about Wi-Fi calling (CNET)
Excerpts: "Instead of using your carrier's network connection, your phone can make a voice call via a Wi-Fi network. In most ways, it's like any other phone call, and you still use regular phone numbers. Wi-Fi calling is especially useful when you're in an area with weak carrier coverage. When Apple included Wi-Fi calling capabilities in its iPhone 6 in 2014, the four major carriers in the US ramped up their support of it. These days, you can find [multiple phone models] that have built-in Wi-Fi calling capabilities."
Learn about Wi-Fi Calling for your smartphone
By cellular service provider
Here is information from major U.S. cellular carriers about Wi-Fi Calling, how to use it and which of their devices support it:
Verizon: http://www.verizonwireless.com/support/wifi-calling-faqs/
T-Mobile: https://www.t-mobile.com/support/coverage/wi-fi-calling-from-t-mobile
Sprint: https://www.sprint.com/en/shop/services/wi-fi-calling.html
US Mobile: https://www.usmobile.com/blog/what-is-wifi-calling/
Google Fi: https://support.google.com/fi/answer/6157793?hl=en
By phone software type
And here is Wi-Fi Calling information from smartphone software makers:
Apple iOS: https://support.apple.com/en-us/HT203032
Google Android: https://support.google.com/phoneapp/answer/2811843?hl=en
In Apple iOS, when Wi-Fi Calling is active, the word "Wi-Fi" appears between the cellular carrier's name and the Wi-Fi signal strength indicator.