How does it know which calls to block?

Nomorobo utilizes a MASSIVE database of known, illegal robocallers, that we've compiled with the help of the FTC, user reports, and our own honey traps.  The blacklist contains over 650,000 robocallers and grows every single day.

If a call gets a match in our database, we grab the call.  If there's no match, we let the call go through. 

We also maintain a growing whitelist of numbers we will never block.