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.
Assuming you only care to hear from a handful of people, open your phone’s Settings, and tap on Notifications. From here, tap on Do Not Disturb and toggle it on. Now, you can tap Allow Exceptions and add the contacts you’d like to get calls from.

This call may be recorded or monitored for quality and training purposes. If you don’t wish this call to be monitored or recorded, then please let the answering machine know when you leave your message.
The main point that we want to drive home with this article is that you shouldn’t overthink your business voicemail greeting. Just keep it short, and state the relevant information.