Tell HN: Forwarding Google Hangouts to phone leads to “Unknown” robocalls

I was getting a lot of robocalls (two to four per day) from "Unknown." Nothing I did seemed to stop the calls. But then a few days ago a client was supposed to call me at a prearranged time, so I picked up the "Unknown" call that came in at that precise moment. The client said he was calling on Google Hangouts. That intrigued me. I checked, and it turned out I'd configured my Hangouts account to forward "calls" to my phone. I unchecked that box and haven't gotten an "Unknown" robocall since.