This. The comments suggesting just disconnecting or disabling the radio are missing that the phone could record stuff & transmit it later when re-enabled. An RF sensor will not stop such recording, either.
Sensors could have small, redundant backups, so hardware input switches might only get a false sense of security. For that matter, even removing the battery is in principle possible to work around (using an extra battery or a capacitor—the power needed to record from a microphone is not very high, for example). We might hope that would not happen due to cost, but what is a few cents out of the price of a modern smartphone?
A soundproof Faraday cage should work, but if someone is that paranoid, they probably need to put the people inside it rather than their phones.
While the standard way of quoting the size is ¼" (& that was surely what was originally specified), that is exactly equal to 6.35 mm, unlike many other claimed (near-)equivalences between metric & U.S. units.
But for exactly that reason, it might as well be called ¼" because it is a more easily memorable number.
I was not aware of that specifically, but that is irrelevant to the point I was making. I was only talking about how people think some brand names are too generic. If they complained that Apple Computer should not be called that because "Apple" is a dictionary word, then I do not see why they would not have said the same about Apple Corps, & vice versa. That the latter sued the former for having a similar name & working in the same area is not the same thing as whether (random people think) "Windows" is too generic a name for a windowing environment or "App Store" for an app store.
It is more like "Android" for an OS or "Oracle" for a database company, unless people actually start using Android to run humanoid robots or asking Oracle to predict the future (which should still not affect their non-genericity with respect to cell phones & ordinary databases, respectively).
Some people have a great ambition: to build something that will last, at least until they've finished building it.