Data as well
This is bigger than voice. Normally the baseband processor (BPC) and OS running on it swaps data with the main OS by reading/writing to some shared memory in RAM, plus some semaphores etc and a couple of hardware interrupts. Unfortunately it's common for no-one to lock down the permissions the BPC has, so that it actually has read/write access to a device's entire RAM. It can then search for crypto keys or data in the clear and exfiltrate them, root the main OS, etc.
The solution for this problem is simple, and it comes to re-evaluating your threat model. Don't treat the BPC and Qualcomm OS running on it as trusted components - treat them as potentially malicious. Limit read/write access from the BPC using the ARM xPUs, specifically the Memory Protection Unit. Unfortunately this is not wholly trivial, as when Qualcomm changes the memory ranges they use then you have to update your memory regions on the protection unit - Qualcomm and memory ranges are a bit like MS-RPC and firewalls...
I guarantee that few phone devs have done the relevant work, as it's a security thing which won't be prioritised, and most trust Qualcomm. Which has been found to be idiotic, if they don't implement any kind of signing checks for BPC updates...
Biting the hand that feeds IT
