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.
To improve a a business voicemail greeting, keep these eight rules at the forefront of the creative process: Avoid turning customers off with overused and impersonal phrases like “your call is very important to us..." Avoid leaving customers unsure by not immediately telling them the business, department, and/or person they’ve reached. Avoid leaving customers confused with too many details and complications; just keep it simple. Avoid messages longer than 25 seconds. Do apologize for being unavailable at the moment. Do invite the caller to leave a message. Do tell the caller when they can expect a return call and actually follow through within that timeframe. Do tell the caller about any applicable alternative options of contact and information- website, live chat, email, social media, or emergency numbers. Voicemail Greetings 101

Hello! Thanks for calling [LinkedPhone. We’re currently tied up assisting other entrepreneurs & small businesses at the moment]. But please leave your name, number, and the reason for your call and we will get back to you as soon as possible. We look forward to working with you. Thank you.
Congratulations! You've recorded a professional voicemail greeting that your callers love. If you want them to keep loving it, you'll need to update regularly. Adding seasonal information and varying your message keeps your greetings fresh, which is especially important for businesses whose clients and customers call in on a regular basis.