How to deal with excessive robocalls.

I recently visited my grandparents and while I was there (maybe about an hour) they received around 10 robocalls on their home phone. They told me that it had been happening all day and while it was rarely that bad its not unusual to get 10 or so a day. In my eyes it seemed like harassment. Their phone rang every 3-4 minutes it seemed. Is there anything that can be done to combat this? I want to try and help them out. This is in PA if thats relevant.