this post was submitted on 12 Sep 2025
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This interference was caused by RF (Radio Frequency) interference from the phone using GSM technology to communicate with the nearest cellphone tower. It would have been heard by any speaker you were close to. They stopped using this tech after 2g. 3g onward, you no longer heard this anymore.
I had Sony Ericsson Bluetooth headphones (12 years old) that somehow still picked those kinds of signals up.
Used as a party trick of late, as it catches most pings within 10+ meters. And breaks up completely when there are emergency services (police and firefighter squads) nearby.
We gave them to a local hacker club for laughs and giggles.
Not just GSM - any phone would do it, GSM was just more noticeable (I had a CDMA phone since 1996, all of them did this until about 2006).
Shouldn't a CDMA phone have caused the issue continuously then? They have a constance radio broadcast
GSM and CDMA don't really work that much differently. Neither has a constant-on radio, they register with a tower and then periodically check for a tower.
The main difference is one uses TDMA (GSM) and the other users CDMA for voice calls. (Time Division Multiplexing vs Carrier Division Multiplexing).
Text messaging is a side-effect of CDMA packet framing - it had to be tacked-on to GSM since it didn't utilize the same connection design.
Yeah I shouldn't have said constant I had that thought wrong. I guess it is better worded as continuosly syncratic rather than time based. I guess I was thinking the regular check-in would have caused the magnetic interference every time instead of just when the connection was amplified.
I thought TDMA would have died with 2g though. We have so many devices now I would think it would be impossible to have time slots for check ins. Sounds like something fun to look into but unfortunately I doubt I'll ever have time to play with that.
Always something new
I don't think it uses time slots for check-in, just for the voice call part. Especially since a phone changing towers wouldn't know what time slot was available.
I assume the interference is caused by a phone increasing it's power output to establish a voice call, which requires much more power than a data keep-alive/get messages connection.
It's been a long time since I did a deep dive though.