Here's another one. We actually wire tapped entire undersea phone lines during the cold war. (Yep, the CIA had developed ways to get submarines down to wires deep in the sea, below depths that the Soviet's thought we could dive to, and then "tap" the entire set of wires they were using for communications.) Did it in Germany too. Heck, we've been tapping lines and monitoring chatter for decades. We had the Japanese codes cracked, and many of the German ones too. (And many other things that nobody is going to talk about even now.)
It's just good intell. (Anyone that says otherwise, is a total fool IMNSHO.)
The fact that we have had NO terrorist plots blowing up stuff here in the USA over the past 5 years is in large part attached to this listening to the phones, and using massive amounts of computer power to sift through all the chatter, and find the stuff that matters, get it to the people who can do something about it, and then act against the terrorists before they blow someone up.