Just an idea - I may stand corrected on this.
In my experience when a call is transferred to an extension via the voice mail. It's treated as a transferred call. My understanding is that only a call directly transferred to the voice mail box, will receive the busy greeting.
Programming Example:
Call transferred from VM - Selector code 2 is programmed as(EXT 401) - caller will hear the "no answer greeting"
Call transferred directly to VM box - Selector code 2 is programmed as (MB 401) - caller will hear the "busy greeting"
