Anyone who travels on business will know about remote voicemail access. The use of a number other than 901 to access voicemail, or sometimes pressing of asterisk
during the greeting message. For old tape based answerphones, 12 rings could trip them into command mode.
Also the pin number used to be set to a default 0000 or 9705 usually, but nowadays it needs to be allocated before the service will work.
So it cuts the chances of the old style hacks that the UK Press is reviewing at the moment.
Of course, email is a whole other story.