Download the "Disconnect Ring Tone" and play it whenever you get a call such as this. Note there is a pause between your picking up your phone before someone answers. This is the computer determining whether it has reached a valid number. If you play the disconnect tone, the computer automatically removes you from their calling directory.
I was a sucker and answered! They were trying to give me a home alarm for free. This is was explained via robot and if I wanted more information I was asked to press 2 and to stop the calls to press 9. When I pressed 9, surprise sursprise, it hung up on me!