You should be able to do this through AD. The Exchange organization should have all the servers from each office in its OU. AD Sites and Services for each office. SMTP Routing setup between each site, etc. Unity will pickup on what's happening in AD and link the sites. VM's will be transferred between Exchange sites. Unity will also be able to see the other Unity servers in AD once everything is connected. You probably need to concentrate on getting AD working correctly through the all the sites. Routing SMTP, Sites and Services. Unity should join right in.