On the Greeting page there are two options. The standard option is to use the default iPhone voicemail message, but you can also record a custom message. If you already have a custom iPhone voicemail message recorded, tap Play to listen to it now. Otherwise tap Custom.
While missed calls aren’t ideal, you can let your caller know you’re still there for them by having a great voicemail greeting. The best business voicemail greetings let your customers (and potential customers) know why you’re not available and how they can best get in contact with your business. They are also short and to the point. You usually want to keep your greeting between 6 and 24 seconds long so callers don’t hang up halfway through.

Finally, and most annoyingly, getting set up with On Voicefeed was pretty tedious. You have to enter basic information, just like almost any other app, which is fine. But then you’re asked to enter a security code that is sent to you by text. At this point, I’m thinking, “Alright, but this better be it.” Then, the app asked me to call a number that was automatically added to my contacts in order to activate the account. Fortunately, that was the end of it, but I was still a little peeved at how long it took to get started. I’m sure that some of those steps are necessary to tap into your voicemail, but finding a way to streamline the process would be much appreciated.
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.