I recently started playing GTA 4 and it has this noise when you're driving in a car with radio on and about to get an sms or call in the game. Made me feel nostalgic.

There was even a song that featured it. https://youtu.be/K20BS-1BPjk

Haha, came here to post that song... Anyways, I still hear this every now and then (the sound, not the song) with my Ford C-max 2006 sound system and iPhone 12 mini, but nowhere near as often as back in the 90's and 00's. Back then I also had a separate vibrating module I'd put in my cell phone pocket (many pockets available at that time, Nokia 3210 did not vibrate), it would always vibrate before any ringtone (self programmed AirWulf) would start (of course).

Why did that stop happening? I remember back in the early 2000s you might even come across a live concert where an incoming text message got caught in the PA and broadcast to the entire audience.

Because we moved from GSM and its original set of frequencies to 3G and (usually) a new set of frequency bands. The noise was caused by a beat frequency between the ring channel and the call/data channel. Changing either the coding scheme (so you don't get anything like a tone as the beat) or the channel separation (so the beat isn't in audio range) causes it to go away.

Nah, the frequencies are stil around there, they're all too high to hear, but GSM used TDM - time division multiplex.

To simplify, up to 7 users (+1 timeslot for signalling) share the same channel/frequency, but each of them is allowed to transmit only 1/8 of the time (even less, due to guard intervals). This basically means, that your phone transmitted for a bit over 0.5 milliseconds (577us), then waited quietly for 7 more intervals like that for other devices to transmit, then your phone transmitted again, and the affected speakers "buzzed" because of these 577ms bursts of power every 4.616 milliseconds (this causes a buzz in the frequency range we can actually hear).

Wow. I've just for the first time ever realised that you might be able to derive the content of the SMS from the specific pattern of interference you would hear on speakers. Like, it seems plausible that is the case. Is that what you mean here by broadcast - or just the fact there was a message was broadcast?

I thought communication between the tower and the device was encrypted?

That's not to say it's not using some archaic weak encryption scheme left in for backwards compatability though.

yes and yes indeed

https://en.m.wikipedia.org/wiki/A5/1

Not really, speakers just reacted to the bursts of power, because the transmission frequency is way too high for us to hear.

GSM uses time division multiplex, which means it shares the 8-timeslot channel with 6 more devices (7 devices + 1 signalling), meaning your phone transmits for 577us, then waits quietly for all the 7 timeslots that other devices use (so, for 4.039 milliseconds - 7x577us), and again transmits for 577us, and again waits. Bursts of power every ~4.6milliseconds make a noise that we can actually hear at a bit over 200Hz.

Good question. Best guess: Better EMI shielding on newer products / different cell frequencies on newer phones. Perhaps combination of both?

It still happens if you lay your phone directly on audio cables with cell data on. You can also see interference happening on high datarate cables or if placed over certain components in a laptop.

It was even better - I knew based on the 'melody' of speakers if I am receiving SMS or a call (and ie started walking to the desk to pick it up)

That's the beat frequency between the ring channel and the call channel. The channel widths and separation were such that the beat frequency was order 1KHz and therefore audible.

Back in ~2001 I saw cute little devices you put in your car and they would light up / blink / dance when it detected the same thing.

They also came in the form of thingies you tied to the lanyard hole on your phone. I had one with my first phone in the mid-00s.

Yea this was when I started leaving my phone on silent and throwing on the desk near the speaker

Good ol moto razr