Why did speakers used to make a strange noise when a phone is about to receive a message? Also, why that doesn’t happen as often now (little to no instances)?
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Two elements. (1) the radio transmitters in phones used to be lousier than they are now, splattering a lot more signal in harmonic and subharmonic sidebands — this was exacerbated by the older digital networks needing a stronger signal; and (2) phones in the U.S. used to use something called Time Domain Multiple Access (TDMA) signaling. Each cell tower would allocate a particular slice of time for each phone in contact with that tower, out of every 100-millisecond-or-so block of time. So your phone would negotiate (a) what time it was (this is a big part of why cell phone time is always dead-on the correct time: your phone *had* to know the exact time, to synchronize with the tower and other phones in the general area) and (b) what times your phone was allowed to talk with the tower, and when it had to shut up to let other phones talk. The buzzing from your speakers was the sound of the phone turning its transmitter on and off very rapidly to use the particular time slice that was allocated for it.
Nowadays essentially all phones use Code Domain Multiple Access — everyone transmits all the time, and mixes the signal with a pseudorandom modulation code, which allows the cell tower to pick out each signal even when several phones are broadcasting at once. That solves a bunch of problems with TDMA, one of which is the buzzing (but there are others also). In particular, your phone’s transmitter tends to switch on and stay on when it is talking with the tower. That still affects nearby circuits, but the effects aren’t in the audio band so they’re not as obtrusive.
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