Action: Disable unified messaging If the on-premises mailboxes you're migrating are enabled for Unified Messaging (UM) and ...

Action: Disable unified messaging  If the on-premises mailboxes you're migrating are enabled for Unified Messaging (UM) and you are performing a cutover or staged migration, you need to disable UM on the mailboxes before you migrate them. You can then enable UM on the mailboxes after the migration is complete. For guidance on moving Unified Messaging-enabled mailboxes from your on-premises Exchange 2007 or Exchange 2010 organization to Office 365, see Plan to Migrate UM-Enabled Mailboxes.