There was a link posted before that Telemarketer's systems are being updated to abide by new laws that mandate that their caller ID be shown and this feature also disables the Telezapper, so it no longer functions. The cost to the telemarketer companies is ~2,000 per seat, so upgrades will not happen fast, but it IS happeneing currently. I am a happy Telezapper user for the last 2+ years and looks like other then the "do not call" list, I will start using the "blocked caller" feature of my caller ID machine soon...
Jim