You can be notified of any server outages - planned or otherwise - by subscribing to the myvrzone-notifcations google group at the following link:
http://groups.google.com/group/myvrzone-notifications
... this group and an associated server monitor are independent of the myvrzone server itself. The monitor will send a notification to the group if the server goes down or comes back up. This is typically the quickest way to be notified so that you can deal with any (potential) guest calls that may occur.