The FTC offer the useful breakdown of steps below.  The full guidance is available here or jump directly to the specific section by clicking the links below.

Got a phone? Then chances are you get calls from scammers. Scammers don’t care if you’re on the National Do Not Call Registry. That’s why your best defense against unwanted calls is call blocking and call labeling. Here’s how to block and report unwanted calls on cell phones and home phones.