I do know after getting on the National Do Not Call List, it slowed the calls down allot. I report each call when they try to sale me something. Each of us reporting helps them stop them. So go here register and never ever call a number back that you do not know, and leaves no message. https://www.consumer.ftc.gov/articles/0108-national-do-not-call-registry
Called, I did not answer, they hung up after 3 rings, as most telemarketers do, Illinois area code. Many states away so suspected telemarketer. I'm on the national, do not call list but evidently this does not stop them. :-(
I do know after getting on the National Do Not Call List, it slowed the calls down allot. I report each call when they try to sale me something. Each of us reporting helps them stop them. So go here register and never ever call a number back that you do not know, and leaves no message. https://www.consumer.ftc.gov/articles/0108-national-do-not-call-registry