In 1986, before mobile phones became ubiquitous, most people used land-lines to communicate from home and work. That year, Congress passed important legislation to translate constitutional privacy protections into the analog age: the Electronic Communications Privacy Act (ECPA). A key part of ECPA requires that the government obtain a warrant in order to listen in on the content of your phone conversations. That's great! But unfortunately, as we moved away from using land-lines to using mobile phones, ECPA suddenly became woefully inadequate.
That's because with mobile phones, law enforcement is often more interested in the transactional details of your phone calls than it is the actual content of your conversations. If a political activist is under FBI surveillance, the agency must get a warrant in order to listen in on her calls. But it can obtain lots of other information about her phone calls without a warrant.
These transactional details include the time and date of the phone call; the GPS coordinates where the call was made; and the numbers dialed. This data can tell the government a lot more about you than can the spoken conversations you have on the phone.
As Susan Landau, a surveillance and technology expert, writes:
Transactional data reveals who spends time together, what an organization's structure is, what business or political deals might be occurring. Reporters were the subject of some of the exigent letter requests. For First Amendment reasons, there are very strict federal guidelines regarding acquiring journalist's phone records, but many of these were disregarded by the FBI. Such broad searches can have a very chilling effect on a reporter's ability to do investigative reporting.