Press UP on the d-pad to bring up your phone, then UP again to bring up the dialpad of the phone. This will allow you to dial this number. After you do this once, the cheat will be in the Cheats menu under Options. This cheat may disable your ability to earn an achievement.

To get free weapons pack #1 (handgun, shotgun, MP5, M4, sniper rifle, RPG, grenades, baseball bat), dial 4865550100 (can be remembered as GUN-555-0100).