You have a phone number gathered by other means or a name with a corresponding phone number. Get a subpeona. Get all the people that person has called. Heck, allow a second tier to in turn get all the people called by those he called in order to "conect the dots". Set up an expedited process - mandate that telecoms have a dedicated liason to provide this data - I imagine they already have a group that answers subpeona requests from law enforcement already.
It sure sounds like a lot of subpeonas. How many tiers? A subpeona for every phone number on the list and every phone number for everyone on the 2nd, 3rd, 4th tier? I don't expect you to have all the answers, just want to point out how complicated it can get. Any these are just phone numbers. And they are looking for patterns not individuals as a first pass. When it comes to individuals, the subpeona process works better I would think.
In other words, you can come up with a quick method to provide the needed data that has due process.
Well the quick method is to run things through a judge but what I think you are suggesting is many, many, many judges or one judge who looks at lots of phone numbers. Anyway, in doing what they are doing and informing Congress I think they are doing "the quick method" out of necessity.
No, one subpeona for all calls tied directly to the phone number in question and for the next level of calls from those numbers.
Like I said, I am willing to bend probable cause in wartime. But not ignore it entirely.