Not a whole heck of a lot to go on here (not even a Unity or Exchange version) so you're not likely to get much useful input here I'm afraid.
Rest assured that Unity isn't going to be randomly deleting a month's worth of messages for random users on the system - we can be sure of that much anyway.
Assuming you're not using tools such as the Message Store Manager (which can be configured to delete messages based on rules if you so choose) I'd take a closer look at folks who have access to that user's mailbox myself...
I am sorry to hear that. That being the case I would suggest either getting a backup solution, and/or enable circular logging for exchange because your transaction logs are accumulating and you could have GIGS AND growing 5MB transaction logs in, by default, your MDBDATA folder. This will continue to grow until your box crashes (bad) or until you do a full backup of the information store.This may not be an issue for a long time depending on disk space and mail activity but a few clicks now could prevent a much bigger issue down the line. I am sure you have this all covered already but for the next one who comes along with a similar issue it was addressed.
SIP traces provide key information in troubleshooting SIP Trunks, SIP
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Please find the attached HTML document, download and open it on your PC.
This provides an easy to use form where you simply answer a few
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CUCM Database Replication is an area in which Cisco customers and
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