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.
Enable the message waiting indicator for your desk phone by checking the box next to Use phone message waiting indicator. 6 Enable Voicemail Notifications 5

We recomend this solution to business that need a phone system, or need to replace an aging analog phone system. This is a fully custom service; therefore, we can build it from the ground up or retro-fit the system to your existing infrastructure. Please call for pricing and consultation.
I can’t come to the phone right now because I’m down in the basement printing up a fresh new batch of 20 dollar bills. If you need any money, or if you just want to check out my handi- work, please leave your name, number, and how much cash you need after the tone. If you’re from the Department of the Treasury, please ignore this message.