I have now completed the OCS R2 Core installation. Voice, video, chat and document sharing (live meeting) now work like a charm. I had some initial problems with the old offline address book (I am replacing OCS 2007 with OCS R2 by removing the old installation and installing a new server) on the clients. But after 24 hours, all clients were synchronized and running properly.
Now I have moved on to installing a new Exchange 2007 SP1 server with the unified messaging role. This server will be added to an already existing Exchange Organization, and I thought it would be an easy task. Not so.....
The installation is pretty easy, and very few things can go wrong. But one of the things that can go wrong was stopping me from installing Exchange onto the server. The only problem was that the error message said: File not found ??? "An error occurred. The error code was 3221684226. The message was The system cannot find the file specified.".
Once again; The problem was a missing prerequisite on the server, but the prerequisite wizard did not catch the missing object. I believe it is because the Windows 2003 and 2008 servers are very different in what features and roles you deploy in a "basic" setup. And the Exchange 2007 wizard was really created for the 2003 server.
In the case of installing Exchange 2007 onto a Windows 2008 server, you also need to install the Remote Server Administration Tools (RSAT) and Active Directory Domain Services Tools (run ServerManagerCmd -i RSAT-ADDS in command prompt). After adding these tools, I had no more problems during the installation of the UM role.