Ok, I'll bite.
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A tiny microphone picks up your voice sound waves and turns them into a series of numbers.
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The sender phone turns these numbers into radio waves and sends them to the nearest radio receiver tower 'cell.' These towers are all over the place, sometimes on top of buildings, other times disguised to look like ugly trees with bad teeth.
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The radio in a phone is not very strong, otherwise it would fry our brain. Those radio waves can only travel a short distance (a few hundred feet to a few miles). To be able to reach farther, there are a series of relays that pass the sound numbers on to the next one until they eventually reach the closest 'cell tower' to the other person.
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These relays send the numbers to each other across wires, where the numbers are turned into electrical or light waves and sent quickly along.
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If the call is to another continent, the sound numbers are turned back into very strong radio waves and sent to a satellite in the sky, which relays it to another satellite far away over earth, which sends it down to a relay in that country, which then keeps it going.
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At the last cell closest to the other person, the numbers are turned back into radio waves and sent to the recipient's weak phone radio over the air. All along, the original voice numbers are kept together, hopefully the exact same ones sent.
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The recipient's phone has a chip that turns those numbers into sound frequencies which are then played into the ear through a small speaker. If everything worked, thouse sound frequencies are similar to the voice of the first speaker, otherwise they would sound like a duck or a sleepy panda.
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All this happens super fast, at a rate of millions or billions of times per second, so we barely hear the delay.
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Reverse the order when the other person speaks.
Now since you're five, you may wonder about all these waves or how radios work. If you ask, my answer would depend on the time of day, what else was on TV, and whether I had finished my beer.
Either way, it would involve magic.
Calvin Edit: ... there's a man inside each 'relay' box with a megaphone...
Would be great if there was some explanation of the temporary unhide and how to go back to normal hide mode. Thanks!