Open the Phone app. Tap the Voicemail tab in the lower right-hand corner of the screen. If Visual Voicemail is not set up on your iPhone, you'll see a button that says, Set Up Now. Tap that button. Enter, then confirm a custom voicemail password. Tap Custom if you want to create your own custom greeting when someone reaches your voicemail box.
Last, make sure you keep this voicemail account up to date, especially if you have some kind of "out of office" message, telling people where you are. Michael Hyatt updates his voicemail message every day-- it may seem excessive, but it lets his callers know that he regularly checks his voicemail, cares about messages he receives, and is likely to get back to them.

You can ask your admin to change the greeting language for everyone in your organization. Your admin should see this topic for instructions: Change the default language for greetings and emails.
For many businesses and professionals, your voicemail greeting is going to be the first point-of-contact for your customers. This is especially true for service businesses, who often rely on their voicemail to collect information from interested parties.