Consider any companies and or government agencies that might have your number on file. Our state DMV has been known for selling information in their database. Then you have something as simple as software that dials any and all numbers.
Numbers that are answered, either manually, or by answering machine, are passed along to other parties of scammers for a price. All it takes is answering (a bite) one unknown number to start a rash of junk calls. It's very similar to fishing, call numbers where there's an opportunity for a bite. They may not get any personal or financial details, or break into your computer, but getting a live phone number is still a win and money in their pocket.