In order to activate the cheat, dial the relevant code as if you were calling a number in-game. Press UP on your d-pad or keyboard, which lets you use the phone. You need to access Niko's phone in order to activate these GTA 4 cheats.
Select the one you want to use from our list. Rockstar Games included many different cheats in Grand Theft Auto 4, each with different effects. Since cheats disable Achievements and Trophies in GTA 4, you'll need to make sure you have a save point to return to once you're done using them.