For a change, this was not a robo-call.  Instead, there was a pleasant sounding woman, in a call center with lots of other people talking around her, on the line.   She said her name, but not the company she was representing. I was expecting a different call at the time, so I didn't say anything and left the phone on mute, until she hung up.  I'm hoping she marks my phone number as an "open line" or "iinvalid" on her sheet.