What Is That Noise?
If you ever do any type of recording or use devices with audio components the day will come where you hear this awful racket. Some call it buzz, some say it sounds like a fax machine and others call it things I won't print here. It sounds like this. What is that noise? It's a BlackBerry® type device or an iPhone (GSM) checking in with master control. It is the sound of digital data being transmitted. At very low levels (when the phone is some distance away), the "buzz" may not be heard, but your audio will sound like there's some "fuzz" on it. The guilty device can be anywhere in the room, possibly passing in the hallway or all the way across the lobby. It is imperative that you monitor your recording because you never know when the problem will occur.

What can you do about it?
Have the device turned OFF. Not on silent or vibrate but OFF. If the device must remain on, then create as much distance between the device and the audio equipment. The greater the distance, the softer the interference.

Cellular phones (GSM , TDMA), iPhones, or a BlackBerry® and the like send out strong electromagnetic (EM) pulses as data messages. They check in with the network for messages and to report their location. CDMA phones and BlackBerry® devices (Verizon) usually do NOT cause any problems. Here is a recent Verizon Q phone incident where the microphone cable was draped directly over the Q phone. Listen. Notice there is interference (at point blank range) but it does not obliterate the content.

Other readings:
That Crazy GSM Buzz
Reducing audio "buzz" in GSM cell phones   A good technical article.
What’s the buzz on GSM?