How do I turn off silence unknown calls on my iPhone 11?

In iOS 18 it's Settings -> Apps -> Phone.

To turn off Silence Unknown Callers, go to Settings > Phone, then scroll down, tap Silence Unknown Callers, and turn "off" the feature.

If left on, calls from unknown numbers are silenced, are sent to voicemail, and appear in the recent calls list.

See Apple Support Write-Up: Silence Unknown Callers