Word! Human intelligence beats the high tech stuff any day. There is simply too much "noise" (useless data) in everyday communications to really sort through. It didn't take high tech to pull off 9/11 or the Madrid bombings.
Can you imagine sorting through a data base of every cell phone call in a day in a metropolitan area. Given my reaction to being forced to listen to one unwanted cell phone call, I would go stark-raving mad trying to listen to a whole cities worth.
The "noise" is not a problem wihen it comes to conventional voice calls. There are only so many international calls live at any one time. I have no inside information on how the NSA does it, but an educated guess would be:
Speaker recognition to identify calls by people they want to listen to. This eliminates the need for tapping specific lines (that's for those lAm3rz at FBI, at the NSA, they listen to wholesale quantities of traffic).
Speech recognition to turn calls into transcripts. They can apply many times the power of the most powerful PC to this task.
Lots of sophisticated filtering to find the interesting transcripts.