As we know for Hot Standby solution, it is for failover if one device is down. And it would synchronize the data periodically.
However there exists the risk of data synchronizing error if you are going to upgrade for the devices in Hot Standby. As the database structure might be different between 2 versions.
Well, no worry. We would show the recommended procedure to upgrade for the Hot Standby devices.
1. Operation time is recommended to be done in low call traffic time. Like after working hour or midnight.
2. Disable Hot Standby mode for both PBX.
3. Execute backup and download backup file for the primary PBX.
4. Upgrade firmware for primary server. After the primary server is up and running, and then, upgrade the secondary server.
5. Enable Hot Standby for both PBX after firmware upgrade.
So far, you would feel free to put the PBX into production again.