The voicemail server is highly customizable, especially when used to reroute external calls rather than just record messages. Even when this is the intended functionality, it should be thoroughly tested to ensure that it is only available to the intended users and only to the for intended destinations, especially if those destinations are external numbers.
Similar care should be taken when using voicemail controls that allow the changing of user, group and system call forwarding destinations, break out numbers, etc.
In general:
Test all call flows with the deliberate intent to find possible issues.
If necessary use the Route Incoming Call action to check whether the caller is internal or external.
Where possible employ PIN codes to restrict the use of call flows.
