Cheat: Spawn Cars
Any time you're outside, break out your cell phone and insert the following phone numbers to unlock the vehicles listed.
* Annihilator (Helicopter) - (359)-555-0100
* Cognoscenti (Car) - (227)-555-0142
* Comet (Car) - (227)-555-0175
* FIB Buffalo (Car) - (227)-555-0100
* Jetmax (Boat) - (938)-555-0100
* NRG-900 (Motorcycle) - (625)-555-0100
* Sanchez (Car) - (625)-555-0150
* Super GT (Car) - (227)-555-0168
* Turismo (Car) - (227)-555-0147
Cheat: Weapon Batch 1
To unlock a baseball bat, grenades, M4, MP5, pistol, RPG and sniper rifle, insert the following number into your cell phone: (486)-555-0100. Keep in mind that this will disable your ability to acquire some achievements.
Cheat: All Secret Locations
Head to the internet café (or wherever you can access the internet) and insert the URL www.whattheydonotwantyoutoknow.com. If inserted properly, you'll be brought to an in-game message board containing maps of secret locations in the game. Check it out!
Cheat: Adjust Your Wanted Level
To get rid of Niko's wanted level, insert the following number into your cell phone: (267)-555-0100. To add stars to Niko's wanted level, insert this number instead: (267)-555-0150. Keep in mind that this will disable your ability to acquire some achievements.
Unlockable: Statue of Happiness Shirt
Head over to the Statue of Happiness and climb up as far as you can go. Head around the left side of the statue from where you entered. When you do this, the game will pause briefly and load a new shirt that your character will be wearing, a nice souvenir for visiting the statue!
Almost always, when you enter a cheat into your cell phone, you can access it automatically from the phone's main menu ("Cheats"), so you don't have to enter these obscure phone numbers more than once.