It had a UHF receiver and for some reason I could listen in on phone calls, but only one side of the conversation. I kept that a secret from everyone.
The old mobile phones worked on 450 MHz without encryption. This is why anyone was able to listen to them. Later the service providers moved it to 900 MHz and to 1800 MHz, both with encryption.
One interesting fact: You can listen to air traffic control with an AM receiver between 108 MHz and 137 MHz. These transmissions are not encrypted.
What's the craziest thing you've heard while messing around with these radios?
The first time I heard the ISS (International Space Station) on 145.800 MHz was one of my most exciting radio experiences. I used an Icom IC-V80E handheld radio. It was around 2010 and 2012. If you have a radioamateur license, and a 2 m (144-146 MHz) transceiver with a directional antenna, then you can try to contact them on the radio. I have not tried that, because you need a directional antenna to be heard on the ISS, and I have not had a directional antenna so far. Receiving them is much easier. Even the factory antenna of a handheld radio is enough to be able to listen to their transmissions. Listening to the ARISSat-1 on 145.950 MHz was similarly very exciting. But that satellite is no longer working nowadays.
That's actually quite interesting. I didn't realize one could tap in to the space station. Don't have much time though; that thing is moving fast, disappears over the horizon, signal cuts out I assume.
Mostly yes, but this depends on the receiver and on the antenna you use, and on the propagation too.