Re: Told to turn off?
It has been studied by Boeing and some instances where there has been strong correlation between use of portable electronic devices and interference with plane systems. Those instances are >20yrs old, and none of them were for phones. However, the report goes on to state:
The laboratory results indicated that the phones not only produce emissions at the operating frequency, but also produce other emissions that fall within airplane communication/navigation frequency bands (automatic direction finder, high frequency, very high frequency [VHF] omni range/locator, and VHF communications and instrument landing system [ILS]). Emissions at the operating frequency were as high as 60 dB over the airplane equipment emission limits, but the other emissions were generally within airplane equipment emission limits. One concern about these other emissions from cell phones is that they may interfere with the operation of an airplane communication or navigation system if the levels are high enough.
I don't know when this report was generated - phones may have moved on since then...
More modern planes are designed with mobile phones in mind, and at least some are perfectly happy for you to have your phone on while the plane is in motion - but I would say that cell-towers do not like phones entering coverage at 200+ mph (take off), and certainly not at 500mph+ (cruising) - but cell towers are not likely to notice a phone up at 30k feet as their antennas don't point upwards in general.
If plane altimeters use the same frequencies as cell towers nearby, then I wouldn't like to rely on that measurement. It's easier to tell folks to turn their phone off than it is to make sure all network cell towers that could conceivably interfere not use that frequency. Or they could have not sold off that frequency, or they could try shifting the frequency of operation of the altimeters (!).
It should be noted that mobiles using ~4GHz spectrum probably won't have the best range. It's not the highest frequency for 5G, but it's not going to have the range of 800MHz at the same power levels.