In the hypothetical in which the phone has been created to act as a bug, surely it would be easy to detect the security plug and disable the microphone for normal uses while leaving it enabled for hidden use. The security plug is only secure if the wiring means that when plugged in, software cannot access the normal mic - if software can access it, it can trick the user into thinking anything.

With the security plug plugged in, actual hardware, the microphone, is short-circuited and cannot produce any signal. That's the whole point.

If you trust the wiring, of course ;-]

But that's my point - if you don't trust the people putting software into your phone, why would you trust the same people with what hardware they put in?

Your argument about not trusting anything can be applied to... anything. It's not hard to make one yourself, so if you're that worried about security, don't use a third party plug.

If you are really that paranoid, take out the battery when you aren't using it :-)

If your phone allows it.

The issue isn't the plug you put in, it's the wiring in the phone - which is fairly hard to make yourself.

And if you don't, you can crack the phone open and look at (much of) the wiring yourself. This is of course a lot harder with modern multi-layer PCBs, but I'd imagine still not impossible. You can at the very least take a multimeter to the microphone pins, and test whether they are indeed shorted or not.

Most (smart) phones have two mics for noise canceling. I would assume this only affects one.