GSM phones are powerful interference generators, more than other, even more powerful, radio transmitters.
Their "galloping" interference is injected into any audio circuit.
It is standard practice to demand to people listening to rock concert to switch off their phones, and I know that in many venues some (obviously illegal) "cellular phone jammers" have been installed to protect the audio quality.
I design and install audio systems for a living, and this problem plagues me.
Screening and shielding is, most of the times, of no help.
Even transformer balancing, ground insulation, and other standard practices are. most of the times, of no help.
Sometimes I question myself: I me, and not the big corporation manufacturing phones, was in the process of testing a GSM phone for CE compliance, will I get an approval?