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.
It works similar to the old answering machine, with the only difference is that the old answering machine will store the voice message in the machine itself, while your phone’s voicemail system will store the voicemail in your service provider server. You can also forward the voicemail message to someone else’s mailbox.

This is a test. This is a test of the Answering Machine Broadcast System. This is only a test.
Voicemails Forever makes editing a voicemail easy and quick. Each edit will become its own mp3 file for sharing or download. This tutorial will guide you through the process.