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. NOTE! Some achievements will be disabled when using the cheats.
Get Full Health and Weapons:
Call GTA-555-0100 (482-555-0100) on your phone.

Get Weapons:
Call GUN-555-0150 (486-555-0150) on your phone.

Get Advanced Weapons:
Call GUN-555-0100 (486-555-0100) on your phone.

Get Full Health:
Call DOC-555-0100 (362-555-0100) on your phone.

Wanted Level Down:
Call COP-555-0100 (267-555-0100) on your phone.

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.

Spawn Jetmax:
Call WET-555-0100 (938-555-0100) on your phone.