There are three possible causes for receiving this error message. The first two causes assume you are running Microsoft Exchange 5.5, the third assumes you are running Exchange 2000:
Administrator Account does not have a Mailbox in Exchange 5.5
To create a mailbox for an administrator, perform the following steps.
Note: The first name and last name can be the same.
Administrator Account is not the Service Account and does not have Privileges to all Mailboxes
To make the administrator account the service account, perform the following steps.
The Anonymous User Account window appears.
Exchange Domain Administrator Must be Configured to Access all Mailboxes
To set privileges to access all user accounts, perform the following steps.
The Exchange System Manager window appears.
The <Your Exchange Server> window appears.
The Properties window appears.
The window displays Name and Permissions panes.
For more details on resolving IP Phone Productivity Service, refer to these documents: