Even with a unique security number, the phone has to have either
an installed GPS locating system that is accurate to up to 50m away
(Saw an article on it in a newspaper, asking if it was a viable
option for parents to track down their children that way, to see
what they may be doing), or the phone has to make a call, where
that way the closest two radio transmitters would respond (to give
mobile phones best coverage) and using that, the police can roughly
pinpoint where the current phone location is at.
However, option one requires you have to signed up for it, and
option two require the phone to make a call/receive a call, which
is unlikely as people who stole the phones would most likely pull
out the sim card and sell off the phone.
And you know that some shops will do "one eye closed" phone
buying...