I have a client who migrated from one Unity 5.0 server to another. Both are VMO with on-box Exchange 2003, however they share the same AD. We used DiRT for the migration, and everything "appeared" to be successful for the first full day after the migration. All external and internal callers could leave VM messages, and the subscribers could retrieve them. However, the second day after the migration, the users could only retrieve VMs from internal callers. We eventually found out the mailstore attribute in AD still contained the old server message store I thought DiRT would change this attribute during restore? I'm trying to confirm this but the documentation I can find only says it changes the Unity attributes with no specific mention of the mail store.
We ended up migrating the Exchange mailboxes from the old server to the new.