Each ext mailbox has 3 greetings that can be used,
No Answer, Busy, After Hours. Sometime people change and the after hours greeting of mailbox does not get changed. If I understand your question, not very clear.
You are saying a call comes in, answered by Auto Attendant, transferred to an ext, ext rings then you answer ext and hear a greeting??? You should be connected to outside caller at that point.
Tell us the exact sequence of events that brings on this Phantom greeting.