Cell phones send and receive signals. These signals can be picked up and cause interference. Older GSM phones have this effect and static can be heard thru speakers. Read about the differences here:
http://www.pcmag.com/article2/0,2817,2407896,00.asp
GSM came first. It's a "time division" system. Calls take turns. Your voice is transformed into digital data, which is given a channel and a time slot, so three calls on one channel look like this: 123123123123. On the other end, the receiver listens only to the assigned time slot and pieces the call back together.
The pulsing of the time division signal created the notorious "GSM buzz," a buzzing sound whenever you put a GSM phone near a speaker. That's mostly gone now, because 3G GSM (as I explain later) isn't a time division technology.
Can't tell ya how many phone calls I have taken asking em how to fix "static noise". My usual answer was:
1. Hang up
2. Turn off ya cell phone
3. Get a feather duster of fluffy towel and skip around the room (never let 2 feet touch the floor at same time) waving the feather duster or towel in circles and passing over everything in the room.
4. Then call me back from a land line
When they called me back and tell me it worked, they ask to explain why I say "Well step 2 solved the problem, step 3 was my "fee" for the phone call. But I overcharged you, we all been laughin our arse's off for the last 10 minutes and that falls into the **priceless** category."