Hackers call voice mail systems looking for mail boxes that have the default password on them, and once they find a mailbox that does they use it and the default password to access the system for international calls. This is easily combated by making sure you change your default password as soon as you get your voice mail box. Today many voice mail providers use a personal piece of information on all new accounts. This helps as the hackers calling random voice mail boxes will not have this information as they don't know who the boxes belong too.
More information on this and other common frauds can be found at: www.fcc.gov