It sounds like an integration problem. Is the number dialed, a normal non vdn extension? does the last name in the station profile match exactly with the name in the Octel mailbox profile. Remember, the Octel needs the last name to come across and the extension, usually after a dot ie., Johnson,M.7997. It can also only be 15 characters.
I have found that if any of these rules are violated, I will get the open tree- the main greeting, or if the caller has a mailbox on the sytem, the caller will hear their own greeting. Still, that's not what seems to be happening to you.
One more thing. I also had one case where the calling extension had to have a mailbox on the system. For example, I had a phone in a cover answer group that had a similar problem when trying to reach a particualr extension/mailbox. I created a mailbox for the extension-even though the extension will not get messages. When I did this, the phone was able to get the correct voicemail greeting they were looking for. It's very strange. It's like the Octel has trouble with some extensions entering it if it has no record of it. I think it's also sporadic. So, maybe this is it.