Voice mail is the corporate alternative to answering machines. Rather than give each employee a separate answering machine, voice mail provides multiple mailboxes on a single machine. Because a voice mail system is nothing more than a programmable computer, phone phreaks quickly found a way to set up their own private voice mailboxes buried within a legitimate voice mailbox system.
The first step in hacking a voice mail system is finding the system's phone number — something a war dialer can do for you. (Many voice mailboxes even have toll-free numbers, so don't forget to scan those numbers too.) If you have legitimate access to a voice mail system, you could practice hacking into it so you have a better idea of what to expect when you work on somebody else's.
When you call a voice mail system, you might have to press a special key, such as * or #. Then a recording will usually ask for a valid mailbox number, typically three or four digits. After choosing a mailbox number, you'll need a password to access the mailbox, play back messages, or record your own messages.
People will usually choose a password that's easy to remember (and easy to guess). Some people base their password on their mailbox number, so try typing the mailbox number itself or backward (if the mailbox number is 2108, try 8012 as the password). Other people might use a password that consists of a repeated number (such as 3333) or a simple series (6789).
Once you manage to guess a password, you'll have free access to the voice mailbox, which means you can play back or erase any stored messages. Of course, if you start erasing somebody's messages, they'll notice fairly quickly and get the system administrator to change the password to lock you out again.
Most voice mail systems always have several empty mailboxes, either leftovers from previous employees or extra capacity for anticipated newcomers. Voice mailbox hackers simply hunt around a voice mailbox system until they find an unused mailbox that they can claim for themselves.
After they've claimed a voice mailbox, hackers can send and retrieve messages from their buddies all over the world. Many companies are providing mailboxes for hackers without even knowing it while other companies ignore or tolerate this minor transgression. As long as the hackers don't mess up the voice mail system for legitimate users, it's often cheaper just to pretend they don't exist on the system at all.