The issue results from incorrectly configured Cisco Unity switch settings and port assignments.
There are two distinct configuration settings within Cisco Unity that can cause this problem. You should check and correct one or both of the following issues.
The Cisco CallManager is not currently selected as the active switch file.
In addition to having the correct switch type configured in the Cisco Unity SA, the ports must also be explicitly defined. (This solution applies only to Cisco Unity with multiple integrations, also sometimes referred to as dual integrations.)