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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 non-existant.

Device Support

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.

Carrier Support

WiFi calling capable phones will have the WiFi calling option available only if the carrier supports it.

Enable WiFi Calling

Device and carrier settings are frequently changed. 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 

https://support.apple.com/en-us/HT203032