On: The FCC Has Fined Robocallers $208M, Collected $6,...

[qubitcoder]

This is already built into iOS, no 3rd-party software required (which would probably just siphon off your contacts).

Just go to: Settings > Phone > Silence Unknown Callers

(Calls from unknown numbers will be silenced, sent to voicemail, and displayed in the Recents list.

Incoming calls will continue to ring from people in your contacts, recent outgoing calls, and Siri Suggestions)