[cabalamat]

You could have a Captcha to force that the person on the calling ewnd is a person and not a computer. That would increase the costs for telemarketeers.

Or the system could ask for information that someone who knows the recipient might know but a telemarketeer might not know, e.g. "What is the 2nd letter of my first name?"

[pavel_lishin]

That still shifts the burden onto everyone else.