I don’t claim to know the origin of the rule, but I can say that about 10 years back, you could hear the whistling in your headset from cellphone use. I’ll specifically reference headset (rx and trx) as opposed to headphones. As there is a lot more critical comm at take off and landing, the rule would seem to make sense. Whether this is still the case today...I don’t know.

Does the FAA does prohibit the use ‘in flight’? it’s odd that anyone would know that and not know that movement towards becoming airborne, including taxing, and run up, has long been interpreted by the FAA to be part of the ‘flight’.

My my first flight as an EXP, was w my wife ORD/FLL. The middle seat (yep, y) was occupied by a non rev FA, who thought it appropriate to moderate our conversation. A brief quip that their opinion wasn’t welcome didn’t stop the matter, so we decided it was time to read our books. I didn’t know who she was, but easy for her to find out who I was. Apparently the FA working the section caught wind of her comments, and made a point of asking what We would like for our free drinks... the middle seater first looked shocked (I don’t recall saying anything nieve or stupid), but then just rolled her eyes.