Factory-Unlocked iPhone 13 Not Getting VoLTE

If your device is supported as you mentioned, and the signal strength is sufficient it should work. Though it's also hit or miss when the other side isn't on VoLTE either. Personally I haven't noticed a huge deal breaker of a difference between VoLTE and regular calls... but if you're up next to the tower, are in a VoLTE-capable LTE coverage area, and it's still not switching you to VoLTE you'll need to reach out to the tech support team when you're at or by the tower so that they can troubleshoot the issue with you.