Make sure to have the latest software update on your phone. This can be a common reason why VoLTE isn't working.
- Tap Settings > Cellular > Cellular Data Options > Voice & Data.
- Enable LTE, 5G Auto, or 5G On.
- Tap the VoLTE toggle to turn it on or off.
- Tap Settings > Connections > Mobile networks.
- Tap the VoLTE calls toggle to turn it on or off.