By default, the global busy prompt and global unavailable prompt are applied to all extensions. If an extension user wants to use her/his personal greetings, you can change the prompts for the extension.
AT&T: If you’re an AT&T customer, you have the option of creating your own basic voicemail. You’ll need to press and hold the 1 key down while you’re on your phone. AT&T will then ask you to set up your greeting via voice prompts.

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.
Tap Record a new greeting. Note: If necessary, delete an existing greeting (limit of 2 greetings) to make room for the new greeting: Tap the Menu key, tap Delete greetings, tap the check box next to the desired greeting, and then tap Delete.