The most extreme example of this I ever encountered was flying SYD-JFK on QF.

The flight made a stop at LAX and passengers continuing to JFK were asked to remain on board. So naturally, I pulled out my cell phone to call my wife. Instantly, the F/A sprinted over to me and got very agitated telling me to immediately turn the phone off.

I complied but asked him what the problem was since we were not only on the ground, but at the gate with the engines shut off.