WiFi calling allows mobile phones to use WiFi networks to place and receive calls even when a cellular connection to the carrier's wireless towers is weak or totally unavailable.
Almost all new phones support WiFi calling, but it must be enabled on every phone. When you upgrade your phone, even if all other settings were restored from backup or your old phone, you must enable WiFi calling on the new device. This is a completely different setting than connecting your phone to WiFi.
WiFi calling capable phones will have the WiFi calling option available only if the carrier supports it.
Device and carrier settings are frequently changed. Please reference your device or carrier's documentation for how to enable WiFi calling. Links to some of these pages are below. If you cannot find the settings on your phone, please contact your wireless carrier and ask them how to enable WiFi calling for your phone.
Apple WiFi calling
AT&T WiFi Calling
Google Fi Phone Information
Sprint WiFi Calling
T-Mobile WiFi calling
Verizon WiFi Calling