Sell a micless phone. Actually, it's a tablet, and if you want it to go into surveillance / phone mode, you plug in your own goddamend mic. Most headsets have one.
Alternatively: a phone whose "off" mode is a battery eject.
Alternatively: a phone whose "off" mode is a battery eject.
I've always found built-in/integrated cameras/mics to represent tremendous negative value.
1. I cannot unambiguously disable them.
2. I cannot upgrade / swap them readily for something better.
Aftermarket/detachable just seems such an obvious alternative.
And yeah, a simpler solution to a hard problem ;-)
Microcurrent devices fed by a small cap or disguised secondary battery might function. But you'd have very limited capabilities.
You might well ask how sure you are that there's not a secondary microphone on the device, to which I can only suggest you cannot be too paranoid.