Most commercial walkie-talkies are not encrypted in any sense; they broadcast the signal plainly, so any similar walkie-talkie can read the signal.

A cellphone is connected to a cell tower to place and maintain the call. The cell tower encodes each cellphone's connection with a specific code to keep the data separate. A phone can only interpret the data that is encoded with their specific code; they will ignore others, so as to not generate extraneous noise on the call.

Several reasons. Encryption is one. Your phone has unique network identifiers that the network uses to connect a voice call or send/recieve data to only your phone.

Personal walkie-talkies work on a smaller range of frequencies and they send out analog signals. These are basically "raw" signals without any special encryption or compression or processing. An analogy would be with light waves, and shining a flashlight towards your neighbor's window. Anyone with eyes who knows which house is going to be sending a message can look at that window and see the light.

Cellphones are on a larger range of frequencies and they send out digital signals. When you talk into the phone, it gets processed into bits (0s and 1s), sort of like recording a sound file. And that takes up space, so the phone then compresses it so it takes up less space (and also reduces the quality of the audio), and then sends that over several varying frequencies so it can all be received at the same time at the destination location, where it then has to be decompressed and converted back into an analog signal (sound). And there are different ways of delivering that signal to the target phone, for instance, it might get cut up over a few different frequencies or sent out along with other calls on the same frequency (just received and decoded by the appropriate recipient). All that means that even if you happen to tune in to the same frequency that someone else's phone is broadcasting at, you're not necessarily keyed in to the same timing and signal that it's sending at, and because it's digital your phone won't actively be decoding whatever it is receiving because it's not on an actual call.

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They all use frequency hopping, in addition to encryption and such. This means that they automatically retune to a new frequency many times per second. A simple tuner can only stay on one frequency. Even if you have one that can hop, it needs to know the pattern to follow in order to track the signal, and the agreement on a pattern between the sender and receiver is kept secret by encryption. The common way to intercept cellphone communications is to have your own device that acts like a cellphone tower, trick the phone into connecting to that, and read off the data after it has been received, while transparently forwarding it to the real cellphone network.

You used to be able to use a scanner to listen to cell phone conversations decades ago before they were encrypted. It was actually kinda fascinating. The type of 850 mhz band scanner you needed was on the more expensive side at the time, but was still the sort of thing you could by at Radio Shack.

I have a pet theory that there's probably a few people who got rich using insider information they gleaned from picking up conversations in the Wall Street area that the participants thought were confidential.