Che_Che_Cole on reddit wrote three years ago this:

simply that we don’t use TDMA anymore, time division multiple access. TDMA was a way for multiple users to share one channel (basically a radio frequency, not unlike a TV channel over the air or tuning your radio to a certain station). It did this by splitting up each user’s signal into short bursts of data. Those bursts/pulses of data are what you heard buzzing in your speaker.

If you’re American you may have noticed back in those days, only Tmobile and ATT did this. They were GSM carriers who used TDMA. Verizon and Sprint used CDMA which was a different technology that did not cause the buzzing speaker because it didn’t transmit in pulses of data.

Newer technologies don’t use TDMA either, so 3G, LTE, now 5G won’t cause a buzz. If you noticed the speaker buzz phenomenon started disappearing in the early 2010s (in the US), that’s why.

https://www.reddit.com/r/explainlikeimfive/comments/slt8j4/comment/hvtdne2/