All plans above the Moose 9 have unlimited free voicemail retrieval and deposits. The Moose 9 plan has unlimited voicemail deposits, but will use part of your monthly minutes to retrieve voicemail messages.
Additionally, when you are prompted to return a call within the voicemail this is not a free feature and is charged at $1 per minute. This is because it is classed as a premium service by Optus.
Moose Mobile recommends you avoid using this feature and simply listen to your received message and then manually return the call through your contacts or via the dial pad.