in the programming, system> music on hold, click on voicemail extension port of one channel and drag in to moh port slot.
you must make sure to remove this extension/SO port of the voicemail from the group.
you will have 4 message recordings for this port and they will run in a loop for moh, so recording in one will loop 1-1-1 or recording in 2 will loop 1-2-1-2- and so on..