Is this a Windows voicemail? If so you can modify the registry.

https://support.microsoft.com/en-us/kb/914387

Here's a tool from MS, called tzedit.exe that makes that process way easier:

https://compu85.homeip.net/stuff/TZEdit.zip

I'd also suggest running an NTP updater on the machine like NetTime.

-J


Jason Perkins
Network Engineer