When LTE coverage deteriorates and Wi-Fi is available, VoLTE calls will be converted to Wi-Fi calls. Wi-Fi must be switched "ON" for this automatic transfer to take place. This implies that if you have Wi-Fi Calling enabled on your smartphone, it will only be used if cellular coverage in your location is poor. If you are on an active VoLTE call and the coverage begins to degrade, the call will attempt to transfer to Wi-Fi when the device is linked to a strong Wi-Fi signal.