On the other hand, a stellar professional voicemail is more than just a way to ensure callers are heard. It’s actually a gateway to encourage recruiters, clients, connections, venders, and other callers to continue forward with the first step in developing a business relationship, which is them making contact. The power of the right voicemail greeting is the caller actually staying on the line to leave that contact information or gain access to an alternative contact point.
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 record greeting, it is mandatory to have a strong internet connection. Alongside, even if the network connection is perfect, then switch to Wi-Fi to Cellular data or vice versa.
You can change the global voicemail greetings for all the extension users or change voicemail greeting for a specific extension. A quick start guide for the administrator who is new to Yeastar S-Series VoIP PBX.