It helps them to find people who they were in contact with and so get deeper into their network. If they have a log of all the telephone calls they are then ab

It helps them to find people who they were in contact with and so get deeper into their network. If they have a log of all the telephone calls they are then able to run a simple "SELECT * from BigTable WHERE 'badphone' = NumberCalled" and then take the investigation from there. I know that's simplified but gives a highly filtered starting point of people of interest. Another reason not to publicise their successes is that may give an indication of how they are tracking and if the terrorists know how they are being tracked they can change how they operate. For example perhaps they always call directory enquiries to get a number/address and if that is the way in it's in the interest of the spies to not let them know that so they can continue to use that chink.