The release of the Unified Messaging server role in Exchange 2007 had many administrators curious about their ability to deploy the features without having an extensive telephony background. Many administrators hesitated, fearing the unknown. However, Exchange 2010's unified messaging capabilities and management features should make administrators reconsider the pros of using the Unified Messaging server role to provide a universal inbox.
Unified messaging is not a replacement for your PBX or your telephony guru. Granted, your legacy PBX may not work with the Unified Messaging server role (although a simple VoIP gateway can often connect the two), and you may want to deploy an IP-PBX, which would eliminate the need for a PBX/VoIP combination. Ultimately, you are going to need trunk lines coming in, connected to some form of PBX to allow for multiple client sharing through extensions, and a telephony expert (who may or may not be you) to manage those items. But you don't require all that to start.
The Exchange 2010 Unified Messaging server role is meant to manage the voice mail side of your organization. Callers can leave a message if you aren't at your desk or if your line is busy, and that message goes to your inbox. You can, of course, access the message though your phone system in the traditional way, but you can also access your calendar, contacts, and other Exchange-related features and make adjustments, all through the phone.
Getting started with unified messaging
You can begin by installing Exchange 2007 or 2010 and making sure to install the Unified Messaging role on a server, either a stand-alone unit or a server that runs other roles (with the exception of the Edge Transport or, if you're using Exchange 2007, a clustered mailbox role). Once the Unified Messaging role is installed, you can begin creating dial plans, UM gateways, UM policies, and the UM auto-attendant (which is your Exchange phone operator). I've posted a few videos on configuring a UM server at my Exclusively Exchange site.