not receiving calls
i have an iphone 8 plus ios 13.3 but i do not receive all of the calls . airplane , do not disturb & phone forwarding is off. I have no idea why i do not receive some calls.
Take your SIM card out and then put it back in.
Check to see if Silence Unknown Callers is turned on under Settings > Phone. If the setting is on, a caller must be listed in your Contacts or Recents. Or they must have shared their number in Mail or sent you a message using that number in order for their call to go through. Add their number to Contacts to ensure the phone rings. Calls from some third-party apps might not go through. Or just turn this option off.
Calls are going through your carrier network, so I highly recommend contacting your carrier on this if none of the above resolves an issue.