Go to any phone and enter the 18 numbers, in no specific order, to unlock 
Keeper of Secrets accomplishment. These can be found throughout the game on 
posters, graffiti, and pretty much anything else. Out of the 25, you only 
have to enter 18 to unlock the accomplishment, after that the rest are useless.

Password - Effect
555-6118 - 1/18
555-1233 - 10/18
555-1037 - 11/18
555-3947 - 12/18
555-1206 - 13/18
555-9562 - 14/18
555-9528 - 15/18
555-7934 - 16/18
555-3285 - 17/18
555-7892 - 18/18
555-9723 - 2/18
555-1847 - 3/18
555-5289 - 4/18
555-6667 - 5/18
555-6205 - 6/18
555-4569 - 7/18
555-7658 - 8/18
555-9985 - 9/18