These guys don't have your number. They have a random generator that picks an exchanges and calls all numbers from -0001 to -9999. If it's answered, the number confirmed as an active number and it hangs up. If you press 1 to go to a "do not call list", in also confirms you are an active number. Your number will then be sold to another telemarketer.
When I get a call that's not programmed into my cell, I reject it. If there's voice mail from someone I need to talk with, I call them back. If not, I google the number. It has always been telemarketers. Wish I knew how to stop it. I now program these numbers into my phone as "do not answer". That way I know not to answer if they call again. Still makin' me mad though.