Here’s a useful article on the Third-Party Doctrine. That doctrine, first articulated by the Supreme Court in Smith v. Maryland, has been the principle behind the notion that what people give to third parties — a phone number, for example — is not “private,” at least for constitutional purposes. A “pen register,” of course, is a crude, weak device for data collection compared to the tools available to the intelligence and law enforcement communities today. (A pen register is an electronic device that records all numbers called from a particular telephone line).
Don’t look for the third-party doctrine to go away anytime soon, but expect it to modified judicially, legislatively, or both.