Please try the below:
1. Turn Airplane Mode on and off, and then dial again.
2. Calls are largely affected by the network. Change the network environment and call again.
3. Clear all background apps to ensure the phone has enough memory.
4. Restart the phone and then enter the app, make sure there is enough memory and try again.
5. Back up your data, go to app settings to clear the cache and try again.
If the issue persists, contact our Customer Support for further assistance.