To enter a cheat code, bring up your phone by pressing Up, then Up again to access the keypad. Once a code has been entered correctly a new Cheats menu selection is available on your phone, below Options, where you can access the cheats without having to enter the phone number every time. You have to play to the mission where Roman gives you a phone to be able to use
it for the cheats. NOTE! Some achievements will be blocked when using the cheats.

Grand Theft Auto IV PC Trainer [ download ] Unlimited Health, 9.999.999 Cash. Made exclusively for Cheat Happens. WRITTEN FOR THE PATCHED (Patch #1) RETAIL VERSION OF THE GAME. May not work with all versions. Read the included readme file with Notepad for important instructions on using the trainer. This trainer features customizable hotkeys. ACCESS: MEMBERS ONLY

Get Full Health and Weapons: Call GTA-555-0100 (482-555-0100) on your phone. (Blocks the Cleaned the Mean Streets achievement) Get Weapons: Call GUN-555-0150 (486-555-0150) on your phone. (Blocks the Cleaned the Mean Streets achievement) Get Advanced Weapons: Call GUN-555-0100 (486-555-0100) on your phone. (Blocks the Cleaned the Mean Streets achievement) Get Full Health: Call DOC-555-0100 (362-555-0100) on your phone. (Blocks the Cleaned the Mean Streets, Finish Him, One Man Army & Walk Free achievements) Wanted Level Down: Call COP-555-0100 (267-555-0100) on your phone. (Blocks the One Man Army & Mean Streets achievements) Wanted Level Up: Call COP-555-0150 (267-555-0150) on your phone. Change Weather: Call HOT-555-0100 (468-555-0100) on your phone. Spawn Cognoscenti: Call CAR-555-0142 (227-555-0142) on your phone. Spawn FBI Buffalo: Call CAR-555-0100 (227-555-0100) on your phone. Spawn Turismo: Call CAR-555-0147 (227-555-0147) on your phone. Spawn Comet: Call CAR-555-0175 (227-555-0175) on your phone. Spawn SuperGT: Call CAR-555-0168 (227-555-0168) on your phone. Spawn NRG900: Call MBK-555-0100 (625-555-0100) on your phone. Spawn Sanchez: Call MBK-555-0150 (625-555-0150) on your phone. Spawn Police Chopper: Call FLY-555-0100 (359-555-0100) on your phone. (Blocks the One Man Army & Walk Free achievements) Spawn Jetmax: Call WET-555-0100 (938-555-0100) on your phone.