[bifel]

The method (at least here in my country) is really simple (and not really a secret): The voicemail can be reached from a special number. This number is easy to derive from the actual phone number (usually inserting one or two digits, depending on the provider). When the "owner" calls the mailbox, he gets the "admin" functionality. Anyone else calling it can only add messages. The rest is just simple call forwarding. So all that is needed is finding out the number of the mail box.

[ilanco]

Not sure if it still works, but in Israel it is as simple as prepending the voice mail prefix to the phone number. Some voice mail boxes can receive faxes, so there is some legitimate use for it.