1. Ensure your phone is in a good signal. If you see No Service or Searching in the status bar at the top left of the screen, it means your device is unable to connect to a cellular network and you will not be able to make or receive calls. In this case, make sure you are in an area with network coverage and make sure there is no network outage.
2. Turn Airplane mode on, wait for the airplay symbol to appear in the status bar, and then turn it off again.
3. Reset Network Settings: Tap Settings > General > Reset > Reset Network Settings.
4. Update your carrier settings: Make sure your iPhone is connected to the Internet > Tap Settings > General > About.
5. Verify blocked phone numbers: go to Settings> Phone> Block. Make sure the phone number you try to call is not on the block list.
6. Try re-inserting the SIM card. Remove and re-insert your SIM card, and make sure it is properly secured in place.
7. We also see some users fix this problem by dialing ##873283# or ##25327# and you can try this if you still fail to make calls after trying the above methods.