” the rule is for as long as the door is open.
There is no such rule. The only rule is a regulation that says electronic devices, except a few specifically mentioned items such as pacemakers and hearing aids, may be used only with permission of the operator of the aircraft. So long as the aircraft is powered up the operator (eg, the airlines) can say no cell phones at all, door or no door. There are electronic systems that need to be checked while at the gate and the airlines may desire no cell phones at that time.
The FCC has an additional rule that says no cell phones may be used in flight. So, once the wheels leave the ground, no cell phones.
There is also a regulation that states no one may interfere with the duties of a flight crew member including flight attendants. The pilot may request them to ensure cell phones are off.
Shame on those 9/11 doomed passengers for breaking the rules. /s