A pen register is an electronic device / process that records or decodes dialing, routing, addressing or signaling information transmitted from a telephone or through internet communications. These systems identify the numbers you dial from your telephone and also the ones received.
Both government agencies and private parties are allowed to use pen register devices to some extent. In 1986 the Electronic Communications Privacy Act (ECPA) was passed to restrict the covert collection of data by private parties and law enforcement agencies to make sure that the information that is gathered is relevant material to an ongoing investigation. In 2001 the USA Patriot Act expanded upon the Privacy Act to restrict that information usage to try to further protect public privacy and ensure the proper usage of pen register systems.
Generally speaking private parties cannot lawfully use pen registers unless the communication is needed to ensure the proper functioning of the business and law enforcement officials are required to obtain a court order authorizing the installation and use of a pen register, trap and trace device anywhere within the United States.
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