Enable Silence Unknown Callers

Robocalls are still a massive problem in the U.S., despite the best efforts of carriers and government bodies to combat them. As of iOS 13, however, you can now send all unknown callers straight to voicemail.

To enable that feature, just head to Settings > Phone and tap Silence Unknown Callers. Any call from a number not in Contacts, Messages or Mail will be sent to your voicemail without ringing your phone. Which is handy, since important unmarked calls will probably leave a voicemail.