This works very well in my experience. I wrote a "captcha/graylist-esque" rule for my asterisk server which intercepts unknown incoming calls with "because we

This works very well in my experience. I wrote a "captcha/graylist-esque" rule for my asterisk server which intercepts unknown incoming calls with "because we don't recognize your phone number, please press one now -- or leave a message after the tone". Pressing one transfers the call to the ring group and whitelists the caller id. For grins, _failing_ to press one results in my office speakerphone playing "sad-trombone-sound.wav" -- how I love to hear that sound.