I accept that I retired before cellphones or glass cockpits really took hold, so can't comment, but do recall a Captain advising me, the Navigator, that his compass was giving obviously erroneous readings, and asking me to go back into the cabin and investigate the First Class baggage rack above rows 2 and 3 both sides. This was where the master compass fluxgate senses ( correct name ? ) were stowed in the sidewall construction on the 707, along with a large notice asking that no baggage containing magnetic material be stowed in these two compartments - ( thinks ? why didn't we just block those compartments off.)
I discovered a large tape recorder, about the size of a Remington portable typewriter - remember those ? - stowed in the Port side locker, belonging to the passenger sat underneath. The magnet as part of the loudspeaker was affecting the compass system. Tape recorder removed and confiscated for the rest of the flight. Problem solved.