Quick Technology Question

There was something in yesterday's Times article about catching the Times Square bomber wanna-be that was very confusing to me.

The young woman in Bridgeport who last month sold Mr. Shahzad the rusting 1993 Nissan Pathfinder prosecutors say he used in the failed attack did not remember his name. But she had his telephone number.
That number was traced back to a prepaid cellular phone purchased by Mr. Shahzad, one that received four calls from Pakistan in the hours before he bought the S.U.V.

How did they know it was his phone, unless he paid by credit card (which, frankly, I wouldn't put past him).  There clearly is way to trace these, despite what I have been led to believe by Jack Bauer.  My buddy Brett offered a vague and drunken hypothesis, but if any of my legion of readers who are too shy to comment has any ideas of how this works, I would really appreciate it.   

EDIT

The Times provokes questions; the Times provides answers.  Apparently the last time Shahzad came back from Pakistan he was given extra screening and had to provide a phone number.  Same one he used to handle other transactions.   This guy really is going to go down as one of the worst terrorists ever.  I don't want to make light of things at all- clearly I am on record as saying that AQAP poses a very real threat- but I think the phrase I am looking for, regarding Shahzad, is: what a maroon.