The Security Mentor

Your cell phone, depending on the brand, can be turned into a bug that will transmit everything that goes on within range of its microphone.

This depends on installing software that changes the way the phone works, for example to turn off the ringer and automatically answer when the attacker calls so that the attacker can listen over the phone.

In the real-life examples I've read about, this has required cooperation from the phone company, so it's not exactly an everyday threat. But if you're a political dissident in a repressive country with a government-owned phone company, either don't carry a cell phone or take out the battery (don't just turn it off) when you're having a sensitive conversation.