Readable with incognito mode to bypass paywall, but if you don't mind some formatting problems, you can also use an archive.is link.
tl;dr The FBI literally ran a company that made graphene-based cellphones targeted at criminals that snagged all conversations and texts.
Cox also tells versions of this on the latest Darknet Diaries and the latest Planet Money. He's making the rounds to push the book, and I admit I'm sold on reading it.