The simple answer is yes. You create Mailboxes, by going to Dashboard > My VoIP > Config > Mailboxes. From here you can create mailboxes and configure everything about them; do you want to receive email notifications, if so to which email address, would you like a custom greeting etc.
Once you've created your mailbox you then chose how you would like to use it. You can either assign it to a SIP User (A person) by going to Dashboard > My VoIP > Users and make it their personal voicemail that gets played if they don't answer their extension number, or you can assign it to any part of a Call Route that you wish.
You set up call routes under Dashboard > My VoIP > Routing > Call Routes. A Call Route would do something like dial a user or group of users for say 30 seconds and then go to your "we are very busy" Mailbox if non-responsive. "Call Routes" are a little like mailboxes, you can create and save as many of them as you want, but you must then go and use them by attaching them to a number under Dashboard > My VoIP > Numbers or using them in an Out of Hour's diary. That way when you call say 0207 111 1234 you are telling that number to follow 'X' call route which in turn finish's with 'X' mailbox.