I have an old Sony amplifier (TA-F117R) next to my desk where I keep my cell phone. Sometimes I put it ontop the amplifier, because it is pervecly alighed with the surface.
When I hang some call up and land the phone close to the amplifier the loud beeping noises started to burst from the speakers, something like the "modem calls" in the old ages of dial-up internet connection.
I suspect the phone - transmitter negotiation is responsible for that. The radi waves transmitted from the cell phone are strong enough to induce parasitic currents in the amplifiers circuits leading to the noise.
What makes me wonder is that this noise is generated only when I hang up the call and put the phone close the amp. I can send/recieve SMS, e-mails and browse the internet using the phone. I have bluetooth, NFC, GPS and Wi-Fi on all the time.
I have Sony Xperia L now, but I could observe the interference with all phones I used to have (Philips Fisio, SonyEricsson K770i, Samsung Galaxy-Y,...) and amplifiers (Old stereo, PC speakers).
What exactly causes such interferences and why SMS and others are transmitted without such interference.
The answer here blames the GSM protocol and the fact 800 - 900 MHz signal interfers with the amplifier's structures. However it dosn't address why SMS and data transfer do not cause such interference.