Automated attendants are used by many companies to augment or replace a switchboard operator. When an automated attendant answers, the caller is generally given several options. A typical greeting is: "Hello, you've reached XYZ Bank. Please press 1 for Auto Loans, 2 for Home Mortgages. If you know the number of the person you are calling, please enter that number now."
If the system is not properly configured, the automated attendant passes the call back to the PBX. The PBX reacts to the digit 9 as a request for a dial tone. The digits 180 become the first numbers of a 1809 call to the Dominican Republic. The 011 string is treated as the first digits of an international call. The hacker then enters the remaining digits of the telephone number and the call is completed. This scenario works the same way with a voice mail system.
Before you set up an automated attendant, restrict transfer out of the messaging system as described in Controlling Call Transfers.