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 spawn the Annihilator (police chopper), dial 3595550100 (can be remembered as FLY-555-0100).