i’m guessing that many people who install an alt OS do so for privacy reasons and from what i understand there still remains a huge security and privacy hole regarding the baseband OS
the problem, as i understand it, is that there’s multiple operating systems running on every phone with a cell radio, one being the baseband/radio OS/firmware and it is a) proprietary, b) insecure, c) has low-level access to hardware (think mic, cameras, location, etc.) and d), shares the same memory used by the user-facing OS - this isn’t terribly unlike the BIOS firmware in a PC
in other words, it apparently doesn’t make a difference what OS you install when the underlying firmware controls the hardware
to my knowledge there’s no way to get around this - there’s no open source radio/baseband firmware because the software has to be approved to use in cellular devices and there’s only two major providers (one is Qualcomm and i forget the other)
this rotten state of affairs has resulted in a few phones that a) don’t run Android, b) use separate memory for the user-facing OS and c), have hardware switches for the camera, cell modem, GPS, etc.
PINEPHONE is one (i believe it isolates the baseband) and Librem is another, but i believe there’s a couple more
Librem goes so far as to offer two versions, the more expensive one being a US version where there is a verified chain of custody during manufacture to avoid tampering and unwanted swapping of components, however it’s currently $2K
As of 2019, all available mobile phones have a proprietary baseband chip (GSM module, cellular modem),[2][3][4] except for the Necuno, which has no such chip and communicates by peer-to-peer VOIP.[5][6] The modem is usually integrated with the system-on-a-chip and the memory.[4] This presents security concerns; baseband attacks can read and alter data on the phone remotely.
The Librem 5 mobile segregates the modem from the system and memory, making it a separate module, a configuration rare in modern cellphones.
another potential option is a phone with no cellular modem that uses WiFi only - WiPhone is kind of a dirt-cheap hackers toy, but there are also full feature phones as well
also see Some of the Best Linux Phones
another option is to do as i did and ditch the bloody thing