You probably recognize the loud buzz a ringing mobile phone often causes in nearby speakers. But where does it come from? Let's dissect the sound and see if it reveals something about the call!
First we need to know a little about how GSM works. A certain cell site (tower) is responsible for handling all the calls made in its coverage area. But the allocated frequency band is very limited. Several phones can talk to the same tower on the same frequency by taking turns. Thus, one GSM phone will only transmit a short burst of digitally encoded speech at a time, and will then wait silently for its next turn. This is called time-division multiple access (TDMA).
Up to 8 phones can fit on a single frequency. They take turns during what's called a TDMA frame. One phone may only transmit once every frame, in its own allocated time slot. One frame lasts 0.004615 seconds, a time slot thus being one eighth of this.
Now, let's zoom into the buzzing sound. Plotted below is its waveform. Not surprisingly, it is also a plot of the voltage variation in the speaker cable.
It looks like voltage is being switched on for a short moment and then cut off for a longer period. You may know from high-school physics that a radio transmitter produces an oscillation in the electromagnetic field, which in turn creates a current (and voltage) in a conductor. So the voltage spikes are probably the periods during which the phone is transmitting. And sure enough, when TDMA timeframes are superimposed onto the waveform, magic happens:
So, what does this reveal about the call going through? Nothing whatsoever, luckily.