Why does a mobile phone interfere with the radio or stereo?

The reason why your phone interferes with your stereo is your stereo is actually a very inefficient radio. It'll convert radio signals into sound signals. This used to happen when I was a kid. I lived right at the bottom of a hill of a really powerful medium wave transmitter. Because the signal was so powerful all sorts of things that wouldn't act as radios will suddenly start behaving like ones. I could pick up Radio 5 on my computer and all sorts of things like that. So basically it's picking up the mobile phone which is transmitting digital data which makes that horrible de-beep and chghghghgh type noises. You can also design the radio inside the phone so that it's designed to ignore the wavelengths which it's transmitting on. So anything about 2 or 3 GHz it will completely ignore them.