Hello -
The Busy and internal greetings can only be recorded from the Active Assistant in Unity 3.1(5), not from the telephone user interface. In Unity 4.0(3) however, those greetings can also be recorded via the TUI. You didn't provide your PBX integration. For example, if Unity is integrated with CallManager, then the Forward on no answer response that Unity gives to the caller is your Standard Greeting, or Closed greeting if called outside a given schedule, or Alternate greeting if one is enabled.
Hope this helps! Ginger