This issue is related to the local carrier network. Some carriers don’t fully support VoLTE or require specific settings for it to function properly.
For your reference, the detailed explanation is as follows:
1. The activated or compatible VoLTE settings vary between regions, causing phone calls to fallback to 2G or 3G;
2. You can resolve this issue by going to [Settings] >> [Mobile networks] >> select your SIM card >> select carrier, and turn off [Automatically select network];
3. For further assistance, please contact the SIM card carrier directly to confirm and resolve any network-related issues.