Smith v. Maryland, abridged, said that we have no privacy interest in the metadata sent to a third party in order to complete a call. In the words of 4th amendment jurisprudence, we don't have a reasonable expectation of privacy in items we voluntarily disclose to third parties. As such, the collection of this information by the government is not a search for 4th amendment purposes, and therefore falls entirely outside 4th amendment protection.
THE ACLU's constitutional argument is a novel one that has not yet become binding precedent on lower courts, call the mosaic theory, though it is gaining ground in some recent Supreme Court decisions. See Orin Kerr's paper on this theory of a search.[2]
[1] (pdf warning) https://www.aclu.org/files/assets/order_granting_governments...
[2](pdf warning) http://repository.law.umich.edu/cgi/viewcontent.cgi?article=...