Telemarketers have been a nuisance to phone owners for nearly as long as landlines have been in existence. According to popular robo-calling prevention company YouMail, in one single month there were 2.4 billion robo-calls in the U.S. This is an increase of about a billion calls over the previous year. According to the New York Times, this resulted in millions of complaints to the Federal Communications Commission (FCC).
It's illegal in the U.S. to robo-call people who haven't agreed to it, but that hasn't stopped the bad guys because it's nearly free. Companies set up a calling station from anywhere in the world. These networks operate over the Internet and use automated systems to dial numbers, sometimes from a database and sometimes blindly trying millions of numbers. Once a person answers the phone with a standard greeting, the program connects them to an actual person to move forward with the sales pitch.
Be aware that you might inadvertently give permission. If you buy things online, note that there is often a checked box giving permission to other companies to market to you. Un-check those boxes. While there isn't a fool-proof way to avoid these robo-calls completely you can start by rejecting any calls from an unrecognized number. Answering tells the robo-caller they have found a legitimate number and they will target more calls to it in the future. Remember, just because the number shows up with a local area code does not mean that it is a local call.