Selector Codes allow you to transfer calls, play announcements or access sub-menus just by pressing 1 digit (usually 1->9).
So say for example, an incoming call is answered by the auto attendant. If the caller presses the number "3"....what happens? If Selector Code #3 is set to, say, play an announcement, extensions 30-39 cannot be direct dialed because the auto attendant has been told to process the call a certain way as soon as the number "3" is pressed.
Sorry if my explanation is somewhat unclear.
Oh, I forgot to ask, which voicemail and revision is being used? (Partner Mail, Partner Voice Messaging, Partner Mail VS or Avaya Partner Messaging)