Some networks charge a flat monthly fee to use voicemail, while others charge for each individual voicemail message, or charge by the minute while you're connected to the voicemail system. Some networks even charge you for simply recording a voicemail message - even if you never bother listening to the message.
“Hey, there! This is [your name]. Please leave me a message with your name, number, and the reason you’re calling. If you also tell me [insert random fact] I’ll be sure to move you to the top of my call list. Have a great day!”

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.
You may need the password you created when you set up your voice mailbox. You'll only need to enter your password in the Visual Voicemail app once, during the initial setup of the service. To limit unauthorized access to your voicemail, consider locking your phone when you aren't using it.