For the last 2+ years I have NOT received any Robo Calls, because of a setting on my iPhone.
NO Apps to install, nothing to monitor, just do this:
Go to Settings:
Go to Phone:
Go to Silence Unknown Callers:
NOW you will ONLY be bothered by people in your Contact List, THAT’S ALL!!
Enjoy!
P.S. You should ALWAYS have Wi-Fi Calling Turned ON!!