i agree, thats cell phone
a side question is does anyone know what makes a phone or some phones do this? i have had it happen only once to me but i have had people on phones near me at almost every show i have taped
I believe it's GSM phones that do this. Don't quote me on this, but I also heard that the interference when there are incoming calls or text are understandable, but the random ones that make no sense are when your digital network cycles through checking to see if it can get a better signal by going analog. A larger metal and concrete venue full of electronics would make sense that your singnal would dimish making this happen more often.
My CellOne phone that was GSM did it all the time, now I have a Verizon one that never makes a sound, but don't know what network it's on. Unfortunately, the main culprit seems to be the flat Nokia ones that every service provider had as their base phone for a while and that a TON of people have. The good news is that with all the bells and whistles on the new phones that don't seem to cause interference, most people should migrate over in the near future.
Also, I think the range is only about 4' for causing interference. At work my phone would interfere with my speaker phone all the time...finally, I realized if I just put my phone behind me on a second desk (in a cubicle so it's not far at all) it would NEVER interfere...even with incoming calls.
Far from scientific or definitive, but that's my $0.02