Telephone metadata includes the phone number of the caller and the person called, as well as other data like the identifier number for international cellphone users, calling card numbers, the time the call was made, and its duration and location. Permission to listen to a phone call requires a separate order, US officials say. But the process allows the metadata to be searched for links to bad actors.
For instance, a phone number plucked from a terrorist’s laptop computer could be compared with phone metadata already stored, and perhaps provide new leads. In a process called “contact chaining,” other numbers associated with that phone number could be tracked in the database to discover other associations. Warrants might then be sought for wiretaps on those phones.