I think (although not sure) that cell phone on planes ban was much less about interference with plane electronics and more about the poor handling of a rapidly moving phone on ground cell networks at the time.

Enough flying phones would cause a significant performance degradation. The issue of handling fast switching was solved, but any "because security" or "because safety" laws tend to stay for a long time.

I fly a lot and haven't head of this at all. Sounds like one of those stories that spread from someone thinking they saw something when in reality it was a crew with a phone checking seats for passengers.

I’ve flown over a thousand times and have rarely used “airplane mode”. I’ve never seen any cabin crew using anything to “detect” cellular activity or even mentioning phones.