Whoever they are has the phone fixed to say it's no longer in service which proves these people are a pest,annoyance,and getting peoples numbers without permission this needs to stop!!
The best thing to do is if you don't recognize the number don't answer. I get countless calls like this at home work & cell. These scammers have a monopoly on phone numbers. It's a pain but don't give them the opportunity to generate even more calls to your number by answering.