eli5: why do fax machines sound like satan himself?

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My job involves calling a lot of people, some of whom are listed under outdated phone numbers. Some of the phone numbers are apparently connected to fax machines now.

Why do they sound like that? It’s alarming. They make these bizarre computerized screeching noises. And they seem to keep going indefinitely until I hang up.

I understand it has to do with the radio transmission (??) but why does the caller need to hear this?

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