Unified messaging involves the integration of two disparate systems--your Microsoft Exchange messaging system and your physical telephony system. As a result, you can combine voicemail, fax capabilities, and e-mail messaging into a single mailbox. Implementing this may seem like a daunting task, but it's easier than you think.
And you may also want to check out the on-demand TechNet Labcast, "Configuring the Microsoft Exchange Server 2007 Unified Messaging Server," for an interactive demonstration.Exchange Server 2007, Messaging, Deployment, Tips and Tricks
Source:→ TechNet Blog