Its super easy. So long as you have voicemail configured in your phone's settings then it will just work. You don't need to do anything special to make it come to life, other than making sure your handset knows how to access your voicemail. In most hardware phones, voicemail access numbers are set per "User/SIP" account and whilst the location of the setting differs from phone to phone, they are normally really easy to find. The easiest way to administer any hardware phone is by using a web browser. They all come with web admin interface's that are a lot easier to use than struggling with the small phone screen and limited buttons. Just determine your phone's IP Address and login to it using a computer thats on the same network by typing that IP into a web browser.
We set your default voicemail access number to 8000, so unless you've changed it in your Dashboard (Dashboard > My VoIP > Voicemail), then this is the number you need to pop in and save to bring your phones voicemail functions to life. One very important note, is that we don't enable voicemail on a User account by default, so make sure you turn on your voicemail by changing that at Dashboard > My VoIP > Users and editing the relevant user account or you'll certainly never get any messages!