The classic cellular base/tower has antennas with a downtilt to limit the interference potential to other cells. However this will not always be the case depending on local topography.
As for the guy in 1A, his phone will not be using the new frequency band if it cannot receive a signal in that band. It may still be using 5G on a lower frequency band, but the interference potential is due to the frequency of the transmission, not the modulation method. 3G or 4G could also cause interference if on the 'wrong' frequency. Also bear in mind that the GPWS antenna is pointing downwards, so relatively immune to signals from inside the plane.