The Xiaomi 15 Ultra supports VoWi-Fi (Voice over Wi-Fi).
For your reference, here is more information about VoWi-Fi:
1. VoWi-Fi is a Wi-Fi calling technology that allows you to make voice calls over a Wi-Fi network instead of relying on traditional cellular networks (such as 3G, 4G or 5G);
2. VoWi-Fi enhances communication by enabling calls in areas with weak or no cellular coverage, reducing roaming costs through Wi-Fi calling, and ensuring seamless switching with VoLTE for uninterrupted calls. It also improves call quality by delivering clearer voice with reduced noise and latency when connected to a stable Wi-Fi network;
3. To use VoWi-Fi, you need a compatible smartphone along with carrier support for the feature. Ensure that VoWi-Fi is enabled in your device and that you have a stable Wi-Fi connection for optimal performance.