There was a 2014 thread on the Tor blog about Android security, where a commenter claimed that one of the Nexus phones had a baseband that was isolaed by a serial link, https://blog.torproject.org/blog/mission-impossible-hardenin...
"Even isolated basebands often have a USB bridge (which can be exploited by pretending to be another type of device with a vulnerable USB driver) and direct access to the microphone ... iirc it's using a high-speed serial link, not USB. The microphone is connected to the audio codec, which is connected to the application processor. Uplink audio is sent from there to the baseband."