[pkcpngwy] Can you turn off your Android phone?

Are Android phones, with their open-source operating systems, sufficiently open that one could, in principle, verify that one's phone is not being used as a covert surveillance device against you (turning on the microphone without ringing)? (It is from the get-go impossible to verify this for the iPhone, which is closed source.)

One would also have to disassemble the phone to see that there are no hardware backdoors circumventing the operating system. Is such a hardware audit possible given phone architectures?

Of course, all these difficulties could be avoided if manufacturers designed phones which the microphone can be mechanically disabled (disconnected electrically) with a physical switch. With flip phones, incorporating such a switch would be easy.

Inspired by the story of KGB spies using analog phone ringers "in reverse" as microphone, one wonders which other components of a cell phone (beyond the microphone) can be used as a microphone. I suspect pressure-sensitive touch screen displays can, and of course the various speakers. The issue, according to the KGB, is the giant voltages necessary to convert a, say, a phone ringer, into a sensitive enough microphone to be useful. Can a cell phone produce such voltages? One might be able to rule out this kind of repurposing by simply looking in the disassembled phone for large capacitors.