International callers

Settings > Phone, and enable Silence Unknown Callers.

Other than that, you can try with apps found in App Store that help manage unwanted calls.

You could also contact your Carrier Support and ask what they do to protect you from spam calls.

Check with your cellular provider. Short of muting all unrecognized callers, or a spam-blocking app, there’s not a good means.

There are some reports of country-level blocking, but i have no way to test and verify that.