According to 18 USCS § 3127 [Title 18. Crimes and Criminal Procedure Part ii. Criminal Procedure Chapter 206. Pen Registers and Trap and Trace Devices] pen register means -
a device or process which records or decodes dialing, routing, addressing, or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business.