Many Internet anti-spam techniques work over the #phone: I have many blocked #spam numbers (after a single call), greylisting (most people call again if I don't pick up if away from the phone), protocol checks (if someone speaks the same "hello" before I reply then I block them as a spammer), a kind of CAPTCHA (if I ask "do you offer in-room photovoltaics?" the spammers hang up, since they do). Same as email, I feel all of these techniques could prevent real helpful humans from contacting me.
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