Hi Ruj,
Thank you for your information. So, from what I understand is that everytime the dirsync run, the msExchUCVoiceMailSettings for shared mailboxes user will be reset from ExchangeHostedVoiceMail=1 to null.
Therefore, we can disable the setting for syncing the setting to null. To do that:
1) Go to DirSync machine
2) Navigate to %Program Files%\Microsoft Online Directory Sync\SYNCBUS\Synchronization Service\UIShell
3) Open miisclient.exe
4) Go to Management Agents tab
5) Right click on SourceAD >> choose Properties...
6) Go to Configuration Attribute Flow
7) Under Object Type:user>> msExchUCVoiceMailSettings>> unchecked Allow Nulls
8) Click OK to save
If there is anything unclear, please feel free to let me know.
Amy