The Wikipedia page talks about the history of pen registers over the last 100 years.

It's a lengthy, complicated page, and I'm not a lawyer... but I think the relevant section that describes their current use is this one, which suggests to me that you do need a warrant:

> For law enforcement agencies to get a pen register approved for surveillance, they must get a court order from a judge. According to 18 U.S.C. § 3123(a)(1), the "court shall enter an ex parte order authorizing the installation and use of a pen register or trap and trace device anywhere within the United States, if the court finds that the attorney for the Government has certified to the court that the information likely to be obtained by such installation and use is relevant to an ongoing criminal investigation".

As I understood it, a court order is not a warrant. A warrant requires probable cause, a court order does not? I may be wrong.