http://www.gamefaqs.com/console/xbox...de/933037.html
a few cell phone cheats and achievement listings.
cool easter egg as well
Cell Phone Passwords
At any time during the game, pull out Niko's phone and dial these numbers for the desired effect.
Please note that cheats will affect missions and achievements.
Password Effect
468-555-0100 Change weather
486-555-0150 Get a different selection of weapons
486-555-0100 Get a selection of weapons
267-555-0150 Raise wanted level
267-555-0100 Remove wanted level
362-555-0100 Restore armour
482-555-0100 Restore health
227-555-0142 Spawn a Cognoscenti
227-555-0175 Spawn a Comet
938-555-0100 Spawn a Jetmax
625-555-0150 Spawn a Sanchez
227-555-0168 Spawn a SuperGT
227-555-0147 Spawn a Turismo
359-555-0100 Spawn an Annihiliator
227-555-0100 Spawn an FIB Buffalo
625-555-0100 Spawn an NRG-900
Easter Eggs
The Heart of Liberty City
First, you'll have to have access to Happiness Island. Once you're able to go there legally, find the Helicopter Tours (which is directly east of Happiness Island) and steal a helicopter. Fly to Happiness Island and over the Statue of Liberty, then jump out of the helicopter at the statue's feet. You should land on the topmost tier of the statue, which is basically a square platform with a door in the center of each side. Run around the platform until you see a door with a plaque on either side of it that reads, "No Hidden Content Here." It may seem like you cannot go through the door, but you can-- it doesn't open, you simply walk THROUGH the door. Inside, you'll find an empty room with a tall ladder. Climb it, and when you reach the top, look up; there is a gigantic beating heart, held in place by chains.
Contributed By: LEFBMIA
Secrets
Easy money
Go to an ATM, then cause a traffic jam to the area. Wait for someone to take money from the ATM, then kill him or her. Make sure to block any nearby roads so the ambulance cannot get to the murdered victim. Take the money they dropped on the ground. Then, run a short distance away, and go back. The money should have respawned on the ground. You can repeat this as many times as desired. You can also kill more people taking their money from the ATM to increase the amount of money that respawns.