There's no single best way to craft an after hours greeting. The most important point is that you equip your customers with what they need to know, and give them an avenue to contact you, whether that's through voicemail, email, or other channels.
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.

Use call answering rules to determine what happens when a caller reaches your voice mail. For example, if you have automatic replies turned on, then you could set up your rule to transfer the caller to a different number. If you have multiple rules, you can specify the order in which they are applied.
Through our website's intuitive message creation tools, we make writing your messages easy! Create company greetings, introductory greetings, announcements, specialty messages, voice prompts, on-hold messages and any other message that you might need to support your phone system. A full library of script examples and guides for best practices are available for you to use.