To listen to your voicemail from an external source or number, you would first need to create a mailbox. To do this head to My Dashboard > VoIP > Config > Mailboxes > Create Mailbox, fill in the required fields and be sure to add a mailbox password!
Once you have created a mailbox, you can then create a call route with your mailbox attached to it.
This can be just your mailbox, or it can call a user first and then go to voicemail, the choice is yours! You can use this video here - What is a Call route? - to help you create a route, with the final (or only) module before saving your route being ‘Send to Voicemail’, selecting your newly created mailbox and then assign that call route to a dedicated number.
You will then be able to dial that number externally and then use ** to skip the greeting and access your voicemail! You can also use the extension field for the route in order to dial straight to voicemail internally, similar to speed dial but for a call route instead!