Different phones may have different steps to enable VoWiFi.
From your phone, click at Settings > Mobile Network > Enable Wi-Fi Calling/VoWiFi
From your phone, click at Settings > Mobile Network > Mobile Data > Wi-Fi Calling > Enable Wi-Fi Calling/VoWiFi
If your phone does not have this option, it may not support Yoodo VoWiFi. Additionally, ensure that your phone is updated with the latest software/firmware.