What is a Pen Register?

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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 08 October 2019
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A pen register is an electronic tool that can be used to capture data regarding all telephone numbers that are dialed from a specific phone line. A tool of this type can be used for various types of surveillance, including monitoring of outbound telephone calls from a specific landline by law enforcement officers. In many nations, the use of devices like the pen register is controlled by privacy laws, with that use restricted to criminal investigations.

In the past, an electronic device of this type could easily be attached to a specific circuit in order to monitor activity that took place with a specific telephone line. In some cases, the service provider would manage the connection, working with law enforcement to capture and transmit the data to the proper authorities. Over time, law enforcement equipment that could be installed and managed by authorities directly came into being. Today, the term has come to refer to not only the original devices, but also various types of software programs and other hardware devices that can be interfaced with a mobile or a landline account to record data regarding numbers dialed from any unit connected with that account.


The use of a pen register in many nations is managed with the use of what is known as privacy of telecommunications laws. Those laws require that in order to tap into the telephone line and capture data, law enforcement must obtain the proper legal authorization to do so. Typically, this activity is limited to persons of interest, such as suspects in a criminal investigation or family members and associates of the suspect. Data captured using some sort of pen register equipment is normally recorded in real-time fashion, with some Internet monitoring technology making it possible to not only capture the dialed number but to also quickly conduct a search to identify the owner of the number dialed. In some cases, it is possible to identify the exact location of the receiving end of the call, a feature that is particularly helpful when a mobile or cell phone number is used.

Businesses may sometimes use a pen register or similar equipment to monitor the outbound calls of employees during business hours. Laws regarding this practice vary, although in many nations there are no laws to prevent a business from monitoring the usage of communication equipment that the business owns. The monitoring within this setting may be used to make sure telemarketing and inside salespeople are making the minimum quota of outbound calls per working hour, or to monitor telephone usage in the event that an employee is suspected of using company resources in a manner that is not authorized.


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