In the legal filing, the government says (in a footnote) that there is no Fifth Amendment issue because the Fifth protects against self-incrimination but that,

In the legal filing, the government says (in a footnote) that there is no Fifth Amendment issue because the Fifth protects against self-incrimination but that, here, no one has been charged with a crime. The government is on a "fishing expedition" -- obtaining a warrant to search the phones so that they can (hopefully) find evidence. As mentioned in the article, it doesn't work that way (they're going about it backwards).