Gotta Have Good Hearing
FBI agents have to be in tip-top condition. That means that every part of them has to be in perfect working order, especially hearing. Interpreting tones, listening to important background noises in audio recordings, and simply hearing people speak over the phone are all extremely important. So, agents have to get their hearing checked every so often in order to make sure they're still up for doing their job.
Are FBI Informants Real?
Yes, the FBI actually uses outside folks as undercover informants. They use informants in order to get through any loopholes that might be harming their investigation. Agents can then "bend" the rules to their liking by using these people who aren't actually with the FBI. If their supervisor isn't intently watching them, then it's basically fair game.