Hi -
Unity has specific design/deployment criteria that must be met, i.e round-trip latency between Unity and Exchange; partner server location etc. You may want to consider Unity Connection 8.5 unified messaging which is not tied to Active Directory sites, Exchange server organization(s) etc. Here is a great resource from 2010 Cisco Live. Check out slide 26 for Unity dependancies - the presentation includes Unity Connection as well!
Sincerely, Ginger