The first thing you’ll need to do before you can setup voicemail is to login to the voice control panel.
If you are unsure how to do so, you can follow these instructions: Login to VoIP Control.
Once logged in, click on Voicemail Boxes, underneath Core Functionality on the left hand menu.
Click on Add New Mailbox.
You can now input all the relevant details, including a unique number which cannot be changed.
After the voicemail box has been added, it will be viewable in the main list.