Another nice tool is CellGuard

https://cellguard.seemoo.de/

It works on iOS devices without any extra hardware, even on non jailbroken devices, by analyzing baseband debug logs exported by the OS.

For what it’s worth, the authors note that since this is installed on a phone, by the time CellGuard has detected a rogue base station, it’s too late anyway.

These spying devices often do permit network traffic to flow through, so if deployed widely these apps could be used to report on where large-scale messing with cellular communications is taking place. The only way to stop this technology is to turn off your phone completely (and opt out of any low-power "find my" networks built into Android and iOS, of course).

iPhone / iOS Find My function doesn’t require cellular data.

https://support.apple.com/en-in/104978#:~:text=If%20Find%20M...

Still lots of devices that have physical SIM cards and would be senseless if the device couldn’t be tracked if the SIM was removed.

I figured it worked more like an AirTag and pinged nearby devices.

That's my point. Apple and Google are using local BLE broadcasts that get uploaded to servers for locating devices. That means ICE can detect/count people in the vicinity by just monitoring the location network signals your devices will emit. For some devices, the location beacon feature will keep working even if you turn them "off".

faraday pouch for added peace of mind.

Hmm, says it works with the Pinephone and Pinephone Pro too, which are very portable, and are just generally devices many more would consider useful (I've thought about buying one at least, just for the open-source-ness of it)