It's attached to the phone, John, and the moment you pick up the phone it gives off an audible beep that somehow tells an automated dialer that this number isn't working. You proceed with your phone call if a human called you, but if a computer calls, it disengages at the sound and supposedly they usually log this number as non-functional. Gets you off of lists.

If you're calling someone with one of those, then you simply hear a brief little high-pitched tone before your conversation starts.

SRS