You can silence all unknown callers, but that may also block callers that you want to receive as well. There are third party apps that you can also try, but they are hit and miss as far as how well they work.
Detect and block spam phone calls - Apple Support
Your cellular provider is the best source for blocking the calls before they even reach your phone. Most of the main stream companies have call blocking settings that can be turned on to stop or at least identify the call as spam. For example TMobile has SpamShield that can be used for free and they also offer a subscription service to give you more control.
The Do Not Call registry has many faults as it requires the caller to honor the request by you and in most cases they are spoofing the calling number so they cannot be identified. You can report callers on that site.
https://www.donotcall.gov