You need to check with the Sim card or cell phone company.
Can you let us know what type of phone you are using?
Try these steps:
- Open Settings.
- Click on SIM Management.
- Select Always Ask for voice calls.
- Deactivate & Activate voice service in SIM card options.
- Go back to the main Settings and click on More.
- Activate or Enable 4G Data connection in phone from here.
Go back and try calling so any contact.
Hope, it will work!