Back in the day, cell phones put out their maximum power the whole time they were on. There were fewer towers, fewer frequencies, and a phone on a plane could take a frequency on all towers within 200 miles. A whole plane of phones left on could eat up every frequency in a 200 mile radius of the plane as it flies.
Nowadays, it's not an issue. Your phone will automatically turn off its transmitter if it detects too many towers. The regulation requiring phones to be turned off in flight is outdated.