Reply #5: Realistically, they can't.

[TheWraith]

They can listen to specific numbers, which they claim come from reliable intelligence (no doubt the same reliable intelligence that gave us reports of terrorists with biotoxins disguised as perfume), but they can't have any real assurance that these numbers are actually connected to terrorists. These numbers allegedly come from cell phones and laptops found in Afghanistan and elsewhere, but knowing how honest these people are, I'd tend to doubt it.

The much more probable method, and what it's been reported that they use, is computerized scanning of ALL calls going in or out of the US, which is a much broader thing. They tap into the big telecommunication trunks and check everything moving over the border, probably recording anything which triggered a hit on a variety of words or word combinations. For instance, if you happened to call somebody in London, and you used words which--while innocuous to you--the NSA views as potential codewords, then your conversation might be recorded.