Jettesen,
you are right it would be quite disconcerting if the captain heard the crackling of the GSM transmission in the headset, but given that the closest passenger is probably 10 feet away it would be his own phone. These "crackles" are picked up by speakers, so if you leave a phone on, in the overhead bin, near the PA speaker it could sound horrific with no impact on speakers and headphones further away. They cause this kind of interference only 2-3 feet around them. My main point is that the "interference" is audible only and has no impact on properly shielded electronic equipment, such as the ones founds in modern aircraft.