A Chicago telephone engineer named Mike Sandman has invented an electronic box that can block telesales calls from companies that use automatic dialling machines (ADM).

These systems can dial hundreds of telephone numbers all at once, waiting for human voices to pick up at the other end. A salesperson is not connected until someone in the household answers the call.

However, if the ADM is given a three-tone signal – which indicates an invalid number has been dialled – the system automatically hangs up and removes the telephone number from its databaseSandman says his box plays these three tones when a phone is answered which fools the ADM, but it will still allow human callers to get through.

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