This issue is due to recent changes made by the local carrier, Singapore Telecom, which require new adaptations in mobile phones.
For your reference, the detailed explanation is as follows:
1. The local carrier has updated its network settings or requirements, which is not yet fully compatible with the VoWi-Fi function on your device;
2. To resolve this issue, please upgrade your phone to the latest software version, which includes optimizations for better compatibility with the carrier's network;
3. For further assistance, please contact Singapore Telecom to confirm whether VoWi-Fi is available for your SIM card and phone model.