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.
A custom phone greeting is a recording that plays automatically when a customer calls. They can be set to play right at the start as a professional welcome, as a voicemail greeting, or even as an away message when you can’t answer. Business phone greetings can give your small business a larger and more professional appearance.

4. "Hello, you've reached [your name and title]. I'm currently out on parental leave until [date]. In the meantime, please direct all phone calls to [alternate contact name] at [phone number] and emails to [email address]. Thanks, and I'll see you in [month you'll be back in the office]."
First you need to know how much time your phone system or provider allows for the duration of the greeting. Some are 30 sec some are longer. Then you need to have a few ideas of what you want to say, or ask your callers.