Cell phones produce a "minute-by-minute account of a person's locations and movements and associations over a long period regardless of what the person is doing at any given moment," the ACLU's Nathan Freed Wessler pointed out in an argument before the Supreme Court.
The ACLU is urging the Supreme Court to rule that the government can't access these records without a warrant.
But the government pointed to a 1979 Supreme Court ruling called Smith v. Maryland.
In that case, the Supreme Court ruled that the government doesn't need to get a warrant to obtain a customer's dialing history because they are merely the business records of the phone company.
Several justices seemed uncomfortable with the potentially Orwellian implications of this position.
Indeed, five years ago, the Supreme Court ruled that it was unconstitutional for the police to attach a GPS tracking device to a suspect's car for almost a month without a warrant.