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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Your phone sends out radio waves like a radio station and your wires are acting like an antenna as everyone is saying.
A big reason you don’t hear it anymore is because of shielding like people say and different frequencies, but the major reason you probably don’t notice is that most speakers nowadays have cables that are sending a digital signal. Sending 0’s and 1’s where your minor interference via radio signals gets filtered out.
When a signal is analog the extremely small amount of interference becomes audible
I normally have a USB cable that sends a digital signal to a sound system, I had to switch to using 3.5mm (aux) output for my PC and that cable has a ton of interference with my phone, which gets amplified before being sent to my headphones. The USB digital signal doesn’t have this issue.
TL;DR
The same way your TV doesn’t go out during a snowstorm anymore, technology has changed and interference like that isn’t an issue for modern methods of transmitting data.
Any metal wire can act as an antenna and pick up radio signals. It just so happens that older phones used a radio frequency that speaker amplifiers are good at picking up, so they would and so you could hear your phone’s radio data.
Modern phones use different frequencies (due to much higher data speeds) so it’s no longer the correct frequency for speakers to pick up.
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It wasn’t the speakers themselves, but rather the amplifier. First of all, back then the frequency phones used to operate at was different (900MHz) and fixed. That particular signal would interfere with the audio signal the amplifier was, well, amplifying.
Your phone is not constantly emitting a signal. That would drain the battery and keep the bandwidths busy. Instead, once it’s registered with a tower, it stays put until that tower emits a signal seeking your phone. So whenever a text message would be received, your phone would “wake up” and confirm its presence to the network in order to receive the message. Also, the interference was also present during calls, but the text message had a specific pattern. If you held your phone next to an amplifier during a call, you’d hear a constant sound.
Nowadays, phones operate on a multitude of bandwidths. Furthermore, newer audio equipment is shielded against intereference, as older equipment was designed before mobile phones were as popular.
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