What is WiFi Calling?
With WiFi Calling you can place a phone call if you have a WiFi connection in an area with little or no cellular coverage. Instead of using your mobile carrier's network connection, you can make voice calls via a WiFi network. That could mean using a WiFi connection you have set up at home, or whatever WiFi hotspot you happen to be on when you're out and about, such as at a cafe or library. In most ways, it's like any other phone call and you still use regular phone numbers.
image courtesy of vodafone.co.uk
Enabling WiFi Calling
All three major US carriers (AT&T, T-Mobile, and Verizon) and prepaid carrier Tracfone provide built-in WiFi calling. See the carrier-specific links below to find out how to enable WiFi Calling for your mobile carrier.
Verizon
https://www.verizonwireless.com/support/wifi-calling-faqs/
AT&T
https://www.att.com/shop/wireless/features/wifi-calling.html
Sprint
Sprint has become T-Mobile; see below.
T-Mobile
https://www.t-mobile.com/offer/wifi-calling-wifi-extenders.html
Tracfone
https://e911-reg.tracfone.com/wps/portal/home/!ut/p/a1/dYzLCsIwFES_JtvcKw2tuAtFKVYEF9J6N5KWNC3WpMRowK_3tROd3RnODBDUQFbdBqPC4KwaX0zpEWdlXkiBJe5WGcr9MhXbLE8QBVRAH-VPJMIayIyueb8dpG2SuQHyutNee371z7oPYVowZBhj5MGrtnNW89adGf7a9O4SoP52YTrZ-0ZX8gEbkSEx/dl5/d5/L2dBISEvZ0FBIS9nQSEh/?_ga=1.232729821.310287348.1473702561