The unified messaging server is a server role in Exchange Server 2007 and Exchange Server 2010 that integrates users' voice mail with Exchange email.
The UM server allows an Exchange organization to store voicemail and faxes along with email, calendars and contacts in users' mailboxes. A unified messaging server also allows users access to voice mail features via smart phones, Microsoft Outlook and Outlook Web App. Administrators can manage the UM server role via the Exchange Management Console (EMC) or the Exchange Management Shell (EMS).
Although the unified messaging server role was officially discontinued in Exchange Server 2013, functionality is still available as it has been split between the mailbox server role and the client access server (CAS) role.