I agree with what @carl_dungeon wrote. If you’re really that paranoid about your privacy then you shouldn’t even use a cellphone in the first place - never mind a smartphone. Look at Richard M Stallman for example - he avoids cellphones completely for this reason.
And that’s because there’s no fully-open cellphone out there, and even if there is, it doesn’t prevent your carrier or government from snooping on you, as long as you’re using GSM communications (which is btw very vulnerable). The moment you’ve decided to carry a cellphone around with you, you’ve signed away any expectations of privacy.
So you either live the Stallman way and avoid cellphones and other modern conveniences, or just say fuck it and use them like everyone else. Otherwise, you’re just a hippocrite, fooling yourself into a false sense of security, because prying open a modern smartphone and checking it’s internals is a completely pointless exercise.