Hay Gyes,Here's a little trick I learned recently. its really interesting:

Many telemarketers use automated equipment to dial multiple numbers simultaneously. These automated machines pass on live calls to humans who then go into the pitch you didn't want to hear. Here's the full list of possibilities the automated machines react to:
The phone is answered, sound is heard from the other end followed by a pause. This scenario is considered a live call, and is passed on to a human to start the pitch. Note that really short telephone answering messages fit this pattern, which is why you may get calls on your machine that go like this: "Hello, Mr. Hogan? ... Mr Hogan? ... Hello?"
The phone is answered, continuous sound is heard from the other end. This scenario is considered an answering machine pickup. The number is marked to be called again, at a different time.
The phone is not answered. The number is marked to be called again, at a different time.
The telephone company plays the "I'm sorry, but the number you've attempted to reach has been disconnected" message. "The number is marked to never be called again"
link: http://www.bythom.com/telemarket.htm
posted by Abd ur Rehman