2. AT&T Wireless: Press and hold the “1” button on the dialer screen of your AT&T wireless device. This command dials you into the AT&T voicemail system. Press “3” to access the Greetings menu.
This tutorial is going to show you how to record a voicemail greeting that you choose. It is a good idea to either write out or think about exactly what you are going to say before you record the message. You will then have the opportunity to play the greeting back before you set it for any incoming calls that go to voicemail.

2. Press and hold the "1" key on the keypad. If it’s your first time setting up voicemail, you’ll likely see a message that says “No voicemail number is stored on the card.” If pressing this button dials your voicemail service, listen to the prompts for instructions on completing the setup process.
On the iPhone, go into voicemail and tap "Greeting". Record a few seconds of something and tap stop. Before you tap save, use iFuntastic or SFTP to navigate to /var/root/Library/Voicemail on your iPhone. You'll see a file called "Greeting. amr". This is the audio you just recorded. Delete that file and replace it with your custom message which should then be renamed to "Greeting.amr". Tap "Play" on the iPhone to verify your message works. If it plays, tap "Save" and your custom message will be uploaded to AT&T's servers.