The FBI dishes out thousands of National Security Letters (NSLs) every year; they can simply be issued by a special agent in charge in a bureau field office, and don’t require judicial review. They allow the Feds to obtain the name, address, and records of any services used – but not the contents of conversations – plus billing records of a person, and forbid the hosting company from telling the subject, meaning those under investigation can’t challenge the decision.
It used to be the case that companies couldn’t even mention the existence of the NSL system for fear of prosecution. However, in 2013 a US district court in San Francisco ruled that such extreme gagging violated the First Amendment. That decision came after Google, and later others, started publishing the number of NSL orders that had been received, in defiance of the law.
In 2015 the Obama administration amended the law to allow companies limited rights to disclose NSL orders, and to set a three-year limit for the gagging order. It also set up a framework for companies to challenge the legitimacy of NSL subpoenas, and it was these changes that caused the appeals court verdict in favor of the government.