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.
The Voicemail to Email feature of Yeastar S-Series VoIP PBX allows extension users to receive voicemail audio files as email attachments and quicken response time when they are out of office.

Website: https://myexamsite.com/view/how-to-change-answering-machine-message-on-vtech-phone
But now I'm going to go ahead and record. So press record sorry I missed your call. Please leave your voicemail after the beep and I'll call you back. Thank you. Press stop and now. I could press play and then if I don't like what I hear, I could press record one more time and when I'm satisfied, I could press save on the top right and it's going to save my greeting and that's all I have to do to set a new voicemail on my iPhone. You could always come back to this page. Press record one more time and replace your voicemail here. You're greeting here, and you should always use someone else's phone to call to see how it sounds from a different device.