I think you're talking about a different class of problems, though. The NSA is going to be focused on larger, sophisticated groups; Chinese government intrusions, multi-state-sponsored terrorism, coordinated Russian attempts to influence U.S. elections. Not the type of thing that's going to get caught in a police interview.
Of course there are some sophisticated adversaries who were probably too smart to use phones even before Snowden, but I’ll venture a guess that the typical terrorism conspiracy is closer in sophistication to a street gang than to the KGB. At least, if there were a clear link between a terrorist plot and and institution full of taxpayer-funded career spooks, it would not be terrorism, just war.
