App

Have you tried opening the Settings app, then tap Phone then see if Silence Unknown Callers is on? If it is, turn it off.

If that is not on, see the "Set up an app to filter and detect spam calls" section of Detect and block spam phone calls - Apple Support for apps that may be blocking calls.

If you mean the setting that blocks all calls except those in Contacts, go to Settings > Phone > Silence Unknown Callers.

See: Detect and block spam phone calls - Apple Support