There's also that pesky little 1979 Supreme Court case, Smith v. Maryland
How many times does it have to be said that Smith v. Maryland does NOT answer the question of this program's legality? That case concerned the constitutionality of pen registers. It had nothing to do with the legality of this program with respect to several federal laws. (Something doesn't have to violate the Constitution to be illegal you know.)
And in that case, investigators had clear evidence that Smith was engaging in harassing calls. That is the problem with the government database - it gets calling data for everyone, not just those with suspected ties to terrorism. And a pen register is a very limited device - whereas modern data mining can do a lot more within the context of a searchable database and the ability to daisy-chain call linkages.
Superseded by the Electronic Communications Privacy Act (ECPA) of 1986.
Wow - interesting case.