Originally Posted by pawarrant
You cant get a warrant for something you don't know is coming. A warrant is the proper way to go if you know what number the person is calling from. If a call comes in that is intercepted from a previously unknown line that reveals a terrorist plot, a warrant can only be obtained beforehand if the NSA had some sort of psychic ability.
If the NSA suspect someone of having links to al qaeda then they would presumably apply for a warrant to monitor their calls.
If a call came in to an entirely unknown person then unless they monitored everyones phone calls they wouldn't intercept it anyway.
So all that would be necessary is to demonstrate to the satisfaction of the judge issuing the warrant that their suspicions were justified. It's just a check and balance surely so that the NSA don't have absolute free reign to monitor peoples calls without reason.
It's not like getting a warrant would take a long time is it. In the UK a warrant can be got 24 hours a day within a matter of minutes (if you are prepared to wake up a magistrate or judge).