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