They have access to it if they threaten/indimidate/blackmail you into giving them access. Dummy phones are a real thing; saw a post today on masto by a company… person (?) who said they keep a stash of clean burner phones for when employees travel through US borders. These are all reasonable, and maybe even CalyxOS’s decoy partition (does it still have that?). The larger problem is that few people will use these things, not even bringing a clean phone. And once they start threatening your family and your long-term safety and freedom, it’s highly likely you’ll give them access, if they know there is any access to be had. Which they increasingly do, because universal surveillance blah blah.