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Americans sick of cold-calling salespeople can try to foil them with a gadget
that is supposed to block telemarketing calls by confusing companies’ automatic
dialling machines. These machines, known as predictive diallers, call hundreds
of numbers but don’t connect them to a salesperson unless someone answers. If
the dialler hears the three tones that designate an invalid number, it
automatically hangs up and removes the number from its database. So Chicago
telephone engineer Mike Sandman has developed an electronic box that plays the
three tones when a phone is answered: this fools the machines, he says, while
human callers get through without trouble.