I believe they are only illegal at the major events. Not at local events. I believe it is because if I have already shot a target I can text the yardage to a buddy that has not shot it yet and give him an advantage.
I copied this from the IBO rules.
"Using a cell phone on the shooting range. Cell phones are only to be used when the group
decides there is an emergency requiring such use. Upon entering a range, all cell phones are to be
switched off.