think I found what happens here:
if you have an extended absence greeting on, callers hear the extended absence greeting, followed by "Your call has been forwarded to automatic voice message system, The mailbox belonging to xxx is full"
if there is not an extended absence greeting on, callers just hear "Your call has been forwarded to automatic voice message system, The mailbox belonging to xxx is full"
and do not hear the normal greeting..
thanks ken.