How that works is simple: there are regulations that force that the microphone used for calling is directly connected to the "baseband", which is under control of the carrier. It has to be, because of AT&T's argument: ONE misbehaving baseband can make cell phones inoperable in an area that's up to a kilometer in diameter. So AT&T's cell towers "need" to be able to send out a signal that permanently disables a phone's transmitter.

Regulations say the baseband MUST control: all wireless signals (including wifi and GPS), all microphones and speakers, and it must be able to disable the camera electrically. It must have a tamper-resistant identifier (IMEI number ... kind of).

Oh, it must allow calling the emergency services. If in this mode, during a call to the emergency services it MUST be able to send the exact GPS position (not just once, continuously) to the emergency services at the request of the emergency services (ie. NOT the user, and carriers must facilitate this)

By the way, it's worse: as you might guess from the purpose, it doesn't matter if your phone is on the "spying" carrier or not, other carriers can send commands to other carriers' phones' basebands (because "get off this frequency" is required: spectrum is shared, even within countries. Since phones may go from one tower to another and be required to vacate frequencies, you need this command). It doesn't even matter if you have a SIM in your phone or not (ever tought that if eSIM works, it must of course be possible for any provider to contact and send instructions to the phone, so it opens up an end-to-end encrypted connection to the javacard that the actual phone cpu cannot intercept). In some phones it doesn't even matter if the phone is on or not (though of course eventually it dies). So "meshtastic" or anything else cannot make a phone safe.

And in practice it's even worse. A lot of phone manufacturers "save on memory" and use the same memory chips for the baseband processor and the central cpu. Which means that it's a little bit cheaper ... and the baseband has access to all the phone memory and all peripherals connected through the memory bus (which is all of them in any recent phone). It may even be the case that these chips are integrated in the cpu (which I believe is the case for recent Apple chips). Oh and the regulations say: if there's a conflict over control over (most) peripherals, including the microphone and speaker, the baseband processor MUST be guaranteed to win that fight.

Oh and because governments demand this, but of course neither fund nor test these devices, they are old, bug-ridden and very insecure. This also means that despite the government requiring that these features be built into phones, governments, carriers and police forces generally do not have the equipment required to actually use these features (though I'm sure the CIA has implement them all). Not even carriers' cell phone towers: they have to pay extra to allow even just frequency sharing ...

Here is an article about baseband and baseband processors.

https://www.extremetech.com/computing/170874-the-secret-seco...