Or it used to be.
Regardless, that is what 'airplane mode' is for. Having the device on but not the radio isn't really about that.
Yes, when the SHTF aircraft comms will work as designed, not even close to the same frequency bands, nor modulation techniques, nor power output levels. Not even a slight chance that cell phones will interfere.
In my minimal experience with airplane crashes[1], the aircraft's electrical system was kaput, and I relayed communications to/from ATC and Fire Dept. via ... wait for it, the pilot's personal cell phone.
[1] I ran an FBO at a tiny municipal airport during college (I re-fueled aircraft). I only participated in a few airplane crashes during my tenure there.
I'm happy to tell you that your concerns are trivial. GSM chirping noises in your headset are merely annoying. There are all sorts of other electrical noise sources in light piston powered aircraft, and if they were ever a suspected cause of crashes, wouldn't be allowed to persist, specifically, alternator noise, strobe light generated noise, magneto noise from unshielded p-leads or plug wires, etc. Use shielded signal cable and noise filters on your intercoms if you don't want to hear it anymore (of course, it wouldn't actually go away).
>always keep at least one of them on and away from the instrument panel
Do you have some reason other than superstition to believe that a cell phone can affect the operation of even your light aircraft instruments? Have you observed your instruments malfunctioning while a cell phone is operating in close proximity, and if so, why would you fly with such obviously unsafe conditions?