Who would have guessed that the best part of your phone, the vibration feature (which more people in movies should use), was a double agent in the making?
Researchers at University of Illinois at Ubana-Champaign figured out that tiny little vibration motor can be used to hear you talking (phone call or not) by interpreting the vibrations your voice makes.
Essentially, the motor isn't very different from a speaker: both vibrate for one purpose or another based on signals they're sent by the phone. And a speaker can be turned into microphones, if you can absorb vibrations and reverse the flow of data.
Put that all together, and you have a very rudimentary bug, stored on your person at all times.
The "technology" isn't great--the quality isn't ideal since it's not designed for this purpose, but it's still clearer than a lot of audio tapes you hear in true crime documentaries.
This method is clever and has potentially scary implications for privacy and security. But if you really wanted to turn a smartphone into a listening device, you probably don’t have to go all this trouble: there’s already a built-in microphone ripe for the hacking.