So the NSA is collecting telephone numbers! But why is the press so coy about pressing for the purpose?
Bush says the database of phone calls is not being used for [data] mining or trolling. But we have no reason to believe what he says.
1. You can look for patterns of activity. For instance, suppose the NSA suspects terrorist cells are active in, say, Chicago. Then you look for numbers of calls from Chicago to Pakistan, or Palestine, or Iran, etc. Compare the proportion of those calls with calls from other cities. As soon as you find statistically significant uplift of calls in one location you lift out those calls and apply closer inspection and analysis. You are now a small step away from actual phone tapping (avoiding the FISA courts, of course!) .
2. The database also contains a staggering web of connected calls. Suppose a particular location in Chicago makes many calls to the Middle East, you can follow up on all his other phone calls (shall we say to other US locations), and then see if any of these make calls to the Middle East. And so forth. The pattern and networks could be very complex and the implications might be quite benign. But as soon as there is good reason to suspect unusual activity you jump straight to phone tapping.
Quite neat, really. But is it ethical or constitutional? That's the debate.