At any in-game phone enter the number and you will unlock a special Darkling.
Password ................... Effect
555-GAME ................... Unlocks Special 2K Darkling

Unlockables
Achievements

Complete each achievement to get the allotted gamerscore.
Unlockable ................. How to Unlock
Anti Air (15) .............. Bring down the Chopper
Anti Hero (25) ............. Complete game on normal setting
Bringing People Together (5) Trap 5 enemies within a Black Hole
Bullet Dodger (20) ......... Finish a multiplayer match with less than 5 deaths (minimum of 4 players)
Butcher (25) ............... Claim 20 Killing Streaks in multiplayer
Cannibal (25) .............. Devour 300 enemies
Completionist (20) ......... Unlock 100 collectables
Contract Killer (25) ....... Be on the winning side in 50 multiplayer matches
Darkness Master (100) ...... Only for those who are completely in tune with the Darkness
Executioner (10) ........... Kill 25 enemies using execution moves
Fashionable (25) ........... Collect all the Darkling outfits
Flag Owner (10) ............ Capture 50 flags in a CTF match
Flag Runner (5) ............ Capture 20 flags in a CTF match
Gatherer (5) ............... Unlock 25 collectables
Ghandi (10) ................ Find a peaceful solution. At least momentarily
Gunner (15) ................ Kill 30 enemies using Darkness Guns
Gunslinger (25) ............ Kill 7 enemies within 15 seconds
Hard to Kill (5) ........... Take enough damage without dying within a multiplayer match
Heart of Gold (50) ......... Help those in need
Henchman (15) .............. Be on the winning side in 20 multiplayer matches
Hoodlum (5) ................ Be on the winning side in a multiplayer match in any of the multiplayer modes
Knuckle, Meet Face (5) ..... Perform a melee kill for the first time
Legendary Dark (35) ........ Complete game on hard setting
Legendary Executioner (20) . Kill 50 enemies using execution moves
Legendary Summoner (25) .... Summon 50 Darklings
Made Man (35) .............. Be on the winning side in 100 multiplayer matches
Mook (5) ................... Be on the winning side in 5 multiplayer matches
Murderer (5) ............... Claim 20 Killing Streaks in multiplayer
Patriot (5) ................ Capture a flag in a CTF match
Picking Up Stuff (5) ....... Unlock a collectable for the first time
Ripper (15) ................ Kill 30 enemies using Demon Arm
Roadkill (10) .............. Kill the workers in the tunnel
Rogue Killer (15) .......... Kill 15 enemies using Creeping Dark
Romantic (10) .............. Real guys stick around for their ladies
Summoner (5) ............... Summon 5 Darklings
Take a Look at the Sky (50)  Take down 6 airplanes.
The Collector (10) ......... Unlock 50 collectables
The Don (45) ............... Be on the winning side in 250 multiplayer matches
The Obsessive Collector (15) Unlock 75 collectables
Up Close and Personal (5) .. Perform an execution move for the first time
Void Bringer (15) .......... Kill 20 enemies using Black Hole

Unlock Collectables

Simply enter the phone number at any payphone from within the game to unlock a collectable and remember there are acheivment points for collecting 1, 25, 50, and 100 collectables. Thats 4 EASY acheivments totalling 40 points, not to mention the hours of searching to find the numbers in game!
Unlockable ................. How to Unlock
Collectable 01 ............. 5554463
Collectable 02 ............. 5554525
Collectable 03 ............. 5554565
Collectable 04 ............. 5554569
Collectable 05 ............. 5554603
Collectable 06 ............. 5554649
Collectable 07 ............. 5554676
Collectable 08 ............. 5554682
Collectable 09 ............. 5554691
Collectable 10 ............. 5556118