It all depends how we define "interference". GSM phones will cause any speaker/headphone to crackle and buzz, but it has no effect on the internacl circuitry of the system they are attached to. As a matter of fact the speaker could be inside a radio that is turned off and it will still make a buzzing sound when a GSM phone, near it, transmits. It is the speaker's coil that is picking up the radio waves. While technically we can still call it intreference, it is happening because the speaker is not sheladed form external radio frequencies, and can't be bay nature of what it is. Any system, be it in a radio or otherwise and CERTANLY, I hope, inside an aircraft would be shielded and not be influenced by a 0.5W transmitter. So while it sounds very nasty the interference is only audible.