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.
The usual problem with interference from GSM cell phones is caused by rectification of the strong RF signal at the semiconductor junctions at the input of the amplifier.
GSM uses Time Domain Multiplexing to share the RF frequency with other users - it only turns the RF signal on for a short time at a 217Hz rate with other users and the base station using the remaining time.
This short relatively high-power RF can be rectified by any semiconductor junctions to cause a slight change in the bias level of the amplifier for the duration of the RF pulse (~0.5ms). This is a buzz at 217Hz that can then get amplified by the amplifier to appear at the speakers.
It is not necessary for the amplifier to have any response at the RF frequency which can be up to about 2GHz.
The cure is to provide filtering to avoid the RF signal getting to the first stage of the amplifier. The filtering may be as simple as a small high-frequency capacitor across the inputs, an RF choke in series or a more complex filter.
Bipolar junction transistors are more sensitive to this effect than FET front ends so it will be affected by which devices are used.
It's not obvious why SMS and data transfers would not cause noise as well - they use the same mechanism for transferring data.