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 use the Zit! service, dial 9485550100 (can be remembered as ZIT-555-0100). Zit! will listen to the song that's playing in your vehicle and then text you the name of the artist and song right away! You can also go to Rockstar Social Club (on the real life Internet) and all of the songs you've called in will be stored in your profile so you can buy them from Amazon!