The decrease in private land lines has driven telemarketers to buy lists of private cell numbers. Software used buy telemarketers is set up wil a local prefix (to your number), it will ring the number to see if 1) it's valid and 2) if someone answers vs vMail. These results are then stored for further marketing campaigns and or sold to other telemarketers.

Here's one brand of software they use:

Best steps are to register at DoNotCallregistry.org and block the numbers on your phone.
Good luck