To use VoWiFi, your device and Proximus must be capable of supporting this functionality. Currently, the following devices are supported:

iPhone 6s, 6s plus, 7, 7 plus, 8, 8 plus, X, Xs, Xs Max, Xr, 11, 11 Pro, 11 Pro Max, SE, SE (2020), SE 5G (2022), 12, 12 mini, 12 Pro, 12 Pro Max, 13, 13 mini, 13 Pro & 13 Pro Max, 14, 14 Plus,14 Pro, 14 Pro Max, 15, 15 Plus, 15 Pro, 15 Pro Max.

To activate this feature, go to Settings > Mobile Network > Wi-Fi Calling and toggle Wi-Fi Calling on this iPhone on or off. If a prompt appears, tap Enable.
If the Wi-Fi Calling option does not appear, go to Settings > Phone. Then restart your device.

Once the feature is activated, your device will place a Wi-Fi call when mobile coverage is too weak.