You need to record your voice mail OGM in the voice mail system. You do this by dialing "*<Extension Number>" . or "#<etension number>" (I am not sure what the default was as I have changed it).
Perhaps, I have not explained how I recorded the OGM.
In System Recordings I entered the extension number, and followed the instructions (*77) to record a message. I then was able to playback the message (*99) and I was able to hear it. I then saved it under a name, and I could see it saved under the System Recordings.
My question is how can I eneble it, so it is played instead of the default one, when that extension is called. When I click on its name there is no setting to enable it.