Cellphones, being a new technology, have been involved in few Fourth Amendment cases, and therefore, there is little guidance as to how to interpret some of the new capabilities within the context of the Fourth Amendment’s warrant requirement.
Police, of course, would prefer to claim it is not an illegal search because it is metadata and not the contents of the call. However, this relies on the now very-dated construction of the type of information provided by this metadata.
The world of 1979 was a very different place. Mechanical switches still controlled most phone calls, and the process of obtaining information from them was laborious and had to be performed by phone company employees.
And because calls could be made from pay phones, much of the information that could be garnered was impersonal and virtually impossible to trace to a particular person.
Today, with a modern smartphone that often never leaves the body of the owner, police using cell tower or "stingray" information can track virtually every step the individual takes with great precision.
The invasiveness is vastly greater than the information from a pen register or a "trap and trace" and most individuals, if asked, would certainly indicate that they had a reasonable expectation of privacy in that data.
Reasonable expectation of privacy is the standard used by the U.S. Supreme Court to determine if a warrant is required. Activity that is carried on in public, such as driving a car that is open to public observation carries no expectation of privacy.
Wired.com, "Cops Need a Warrant to Grab Your Cell Tower Data, Florida Court Rules," Kim Zetter, October 17, 2104