This problem occurs if user the user account does not have the proper permissions to launch CallViewer. The account logged may not be associated to any subscriber.
To overcome this problem, log in as the UnityAdmin, and make sure to log into the domain, and not the local system.
The user needs to be logged in as an account associated with a Cisco Unity subscriber for Call Viewer to work.
You can also overcome this problem by running Grantunityaccess Utility, which is located at C:\commserver\grantunityaccess.exe. When you run this utility with no options, the instructions are displayed. The normal use of this tool is to provide the domain/alias of the account which is to have access to the SA, and then from which account to copy those rights (for example, - grantunityaccess -u domain\). This adds the account to the tool.