Personalize your Voicemail
Navigate to the Zoom web portal to customize your profile | https://gemaire.zoom.us
Click on Phone on the left blade to select the Zoom Phone options. It will then default to the Settings Tab

When you scroll down you will find call handling settings including "Greeting and Menu: Leave or check voicemial"

Choose "Edit" and "Add Audio"

Once you are in the "Add Audio" settings, you can choose to type out your greeting and use an automated voice or you can choose to record by computer if you want your personal voice. When completed, just choose the Add the button.
That will then Add your greeting to your extension.