Users can listen to the messages that were left before the upgrade. New messages can still be left (using Subscriber) but will be available after the upgrade.
Refer to here for detailed explanation:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/10x/install_upgrade/guide/10xcuciumgx/10xcuciumg030.html#pgfId-753858
Status of Unity Connection Cluster During RU
During the refresh upgrade, the publisher server is completely disabled for the entire duration of upgrade but the subscriber server continues to provide services to users and callers.
- If the phone system is routing calls to the subscriber server, outside callers and Unity Connection users can leave voice messages but the messages are not immediately delivered to user mailboxes. During switch version on the subscriber server in a cluster, messages that were left on the subscriber server are copied to the publisher server and delivered to user mailboxes.
- Unity Connection users can use the telephone user interface (touchtone conversation) to listen to messages that are left before you start the upgrade. However, users cannot listen to messages that are left during the upgrade. But can access those messages after both the servers in the cluster are successfully upgraded.
- Unity Connection may not retain the status of messages. For example, if a user plays a message during the upgrade, the message may be marked as new again after the upgrade. Likewise, if a user deletes a message during the upgrade, the message may reappear after the upgrade.
- During an upgrade, users can access Unity Connection using clients, such as ViewMail for Outlook and Web Inbox other than the telephone user interface (TUI). During the upgrade, publisher will display “message could not be loaded” for web inbox.
- Administrator users cannot make configuration changes using any of the administration applications, such as Cisco Unity Connection Administration and Cisco Unified Operating System Administration since the administration applications are disabled for both publisher and subscriber during an upgrade.
- Intrasite, intersite or HTTPS networking with other servers is disabled for the duration of the upgrade. Directory changes made on the other servers in the network are not replicated to the server or cluster until the switch version is complete.
