Cisco CallManager tries only for the first listed voice mail port and returns with a busy signal when a second call arrives. However, when the user manually dials the second voice mail port, it works fine.
This problem occurs when Cisco Messaging Interface is installed and running on Cisco CallManager.
Note: Cisco Messaging Interface is not required for Cisco Unity.
To resolve this issue, disable the Cisco Messaging Interface service running on Cisco CallManager.