4G is on a different frequency, it's out of the audible range.
The reason you used to hear it is that a speaker turns electromagnetic vibration, so the back and forth movement of electrons into mechanical vibration, so the back and forth movement of sound.
2G was on a frequency that you could hear, so when the wires in your speakers picked it up like an antenna, your speakers played it back. 4G is much higher pitched, so it's still there, it's just so high you can't hear it anymore.
Edit: Read Milkyway's comment, they sound like they know more about this than I do. It's not the frequency but the amplitude.
No, the frequency isn't necessarily all that different. Some 4G Bands are even below typical 2G/GSM Bands.
The major difference is how the multiple access and Modulation is done.
2G uses short, narrowband, high power bursts. It's the interval of these bursts that causes the interference, not the carrier frequency.
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.
You can always say: 'I'm reading a book or watch series x atm, why what are you up to / why do you wanna know?'
Choose a book or series you know by heart and you always have an alibi.
If they are the kind who asks are you free on day x? Without stating a reason ask 'why do you wanna know?' Or say 'I think I've got plans that day why?'
It sounds like something Jerry would say: "You know these people, the Phone Trappers. They ask you what you're doing, and the second you say you're free—bam!—they trap you into a call. It’s a trap! They lure you in with a text, then they pounce with the phone."