Then, when you get stopped by the cops you have to explain why your phone is wrapped in foil. For which there can only be one possible reason. You might as well also carry a set of lock picks, crowbar and an acetylene torch, perhaps with a set of scales so that you can be processed that bit easier...
You can also create a Conspiracy theorist persona. Keep asking questions online with a semi anonymous account about the government excess, about the moon, the aliens, the freemasons etc.
Then if in court you can try to argue that for a tin foil hat like you, the actual tin foil cell phone protection isn't an indication of anything. And much less suspicious.
Not sure how it could be fully applied if in the police interrogation room having been apprehended in a dark alleyway with two large bin bags full of steaming skunk weed, freshly purchased off a Vietnamese gentleman insistent on counting every note of that £20000 just handed to him. Maybe that is just an edge case though.
Perhaps a better idea (and market opportunity) could be a 'walkers rucksack' that has a pocket for your cellphone. This pocket could be lined specifically to act as a Faraday Cage, explicitly so that you can have easy access to your phone for maps etc., yet be fairly certain that your day strolling in the hills will not be interrupted by the office, the wife and other cold callers. Such a bag could be plausibly denied in a way that plain old tin foil could not be.
