So, I made a prior post but deleted it. Here's some info on prison communication.
Transmitters
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This transmitter was made in a prison in Germany. What inmates will do is they'll take a prison FM radio from the commissary that should look like this:
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And they cannibalize it into a transmitter so that they can eavesdrop on guards and communicate remotely with other inmates. It's essentially a prison cell phone.
Computers
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This computer can be made out of paperclips. It's called the paperclip computer and it's based off a 1960s book telling you how to make it. Seen below.
It is by far the simplest digital computer ever made that can be made out of some everyday items.
Letters
Inmates have also been known to use cryptography to communicate with others or the outside world. One method they use is writing their letters using water as invisible ink, which unlike piss, milk, sperm and citrus juice, can't be detected using UV light. You have to use iodine vapor to reveal the writings. This was actually used by the Nazis during WWII.