I have done some research on this and I think the issue is because the message class is just
IPM.Note and not IPM.Note.Custom.Cisco.Unity.Voice when I check the message class field in my outlook inbox
however when I check the message header
X-CiscoUnity-MessageType: SynchVoice
Content-Type: audio/wav; name=VoiceMessage.wav
Content-Disposition: inline; filename=VoiceMessage.wav; voice=Voice-Message
X-CiscoUnity-DbMessageId: 75a4d2e4-d220-4297-be60-52c7e9a70b7e
Content-Transfer-Encoding: base64
x-ciscounity-messageduration:4390
x-CiscoUnity-SyncTime: 29-Oct-2012 08:01:36 +0000
Content-Class: urn:content-class:custom.Cisco.Unity.Voice.
the content class is custom.Cisco.Unity.Voice.
how is it changing then?