Please try the below:
1. Turn Airplane Mode on and off, and then dial again.
2. Go to settings > Network & internet > SIMs > Enable 4G Calling or disable 4G Calling to try again.
3. Calls are largely affected by the network. Change the network environment and call again.
4. Try a different SIM card.
5. Update your phone to the latest version.
6. Back up your phone data and restore the phone to factory settings.
If the issue persists, contact our Customer Support for further assistance.