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.
For example under Tools, Options, Calls, there is not a specific option for voicemail. There is only an option for Forwarding and Voicemail.

With social media, blogs, email marketing, and yes…voicemail greetings, businesses have lots of opportunities to make an impression. But it’s important to make the right impression. Callers will make inferences on what you say and how you say it, and you don't always get another chance to make a positive impact. You want prospective customers to leave a message. You want them to get a great first impression of your business. So, you need professional voicemail greetings. Here’s how you get them…
Make sure that you set up your voicemail. To check, dial your own phone number from your iPhone and leave yourself a message. Call your iPhone from another phone and try to leave yourself a voicemail. Go to the keypad and press and hold on the 1, or press the Call Voicemail button. This dials voicemail. Check if you have any missed messages.