It does work without OCS (hence Exchange was released one year earlier than OCS)
But integration is required if you want to call Exchange UM via OCS (Enterprise Voice deployment guide)
http://www.microsoft.com/DownLoads/details.aspx?FamilyID=24e72dac-2b26-4f43-bba2-60488f2aca8d&displaylang=en
- Belgian Exchange Community : http://www.pro-exchange.be -