You will have to run Exch 2003 off box if using voicemail only solution. I think starting with Unity 5.0 there is support for Windo2003/Exch2003 on box (I may be incorrect, i will verify this). I am sure running Exch 2003 on box will work but its not TAC supported.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus