I got called 5:29 pm yesterday, 1.31.16. St Louis area. I didn't answer and they didn't leave a message. Gindy is right but the "companies" supposedly need a list to look at so they know who NOT to call by law so the law doesn't come after them. But the government usually says they don't have enough inspectors or time or lack of money, etc., to go after them unless they really get hundreds of complaints about a company or a telephone number they can actually track down. Most of them buy an internet gadget that gives you a fake number ("spoofing"). Of course, that means the "not legitimate" companies could call hoping you believe they are really a charity or political or surveying or the other exceptions or hope you will do business with them because there's supposed to be less competition!!!
And if it's a "fake" exchange ("799") like Gindy says you can't tell me the phone companies can't make them "inactive and unusable" so they can't be used!!!!