A lot of rubbish
A lot of rubbish is spouted here from some folk that know nothing about EMI in modern kit. Cellphones should not 'interfere' with any modern aircraft receiver.
No avionics operates on cellphone frequencies
No cellphone operates on avionics frequencies
Therefore there are only two main possibilities
The airborne receiver has spurious response on cell phone frequencies or it is overloaded by the cellphone either at RF, IF or even in the audio chain.
The option is to make a better receiver, adequately filtered.
or
The cellphone has a spurious output, a spur that is slap bang on a used avionic frequency. This may be more likely as they are cheaply made but phones are certified under FCC regulations to not produce out of band frequencies above a certain level.
The possibility of the second option is low IMHO due to the nature of the emissions from cellphones. Therefore Airbus et al need to spend a few more pennies adding some caps, ferrite cores, stub filters or whatever to stop their avionics responding to out of band signals.
Simple.