[throwawaymobule]

This also exists (existed?) for mobile numbers, and was used by newspapers to 'hack' certain celebrities' voicemail. Including some cases where they deleted existing messages when the mailbox was full, so they could get more.

[hermitcrab]

I thought they just called their phones when they weren't in and used the default voicemail PIN (which most people don't change) to access their messages.

[throwawaymobule]

either default pin or guessed based on publicly available info. calling from their line was also done to get at more mailboxes.