VoWiFi (Voice over Wi-Fi), also known as Wi-Fi Calling. It is a feature in mobile networks that allows users to make and receive voice calls over a Wi-Fi network, instead of the traditional cellular network.

This can happen when users have a strong Wi-Fi connection, but weak or no cellular coverage.

So that it enhances the overall user experience, by providing reliable voice services in areas with limited cellular coverage.

Typically, the VoWiFi UE connects through the Evolved Packet Data Gateway (ePDG) to access the 4G Evolved Packet Core (EPC).

The ePDG authenticates and authorizes the UE. After that it forwards the user's VoWiFi traffic (both voice and signaling) from the Wi-Fi network, to the appropriate PGW (PDN Gateway) in the EPC.

So that the call can be completed through the IMS network that exists within the mobile network.

Users need to have a compatible device, and enable the VoWiFi feature, in order to be able to use this feature.