I had this problem and the only fix that I could do was change the extension number. We are using 0700 extension for our unity voice mail server and I had a users that had extensions 0701, 0702, 0703 and their voice mail boxes would not work. But when I move them to 0721, 0722, 0723 the voice mail boxes work just fine. I am guesting that Unity voice mail server reserves 10 lines when you setup the extension to communicate with it. Example if I use 0600 as my extension for my voice mail server then I can not have any voice mail boxes from 0601 through 0609. This seem to be on the voice mail server side of the house. You can have these extensions in the call manager but you can not give them voice mail box.