I've been spending a lot of time digging into the guts of Exchange Server 2007's unified messaging (UM) server role, for a couple of reasons. First, I think it's an interesting set of technologies that, for the most part, haven't been available to most Exchange sites. Second, Microsoft so far hasn't said much publicly about how UM works, and the best way to get experience with a technology is to play with it. Third, I could really benefit from some of the planned UM feature set, so I'm eager to deploy it at my office.

The result of all this digging has been rewarding; I've learned a lot about how Exchange 2007 implements its UM role, what UM can and can't do, and how it all works.

The first thing you should know is that in Exchange 2007, the UM service set is implemented by the Unified Messaging server role. The UM server is responsible for delivering several key services: - It links the physical telephone system (including internal phone lines and trunks that connect to the public switched telephone network) to the Exchange infrastructure. - It applies dial plans, which are sets of rules that control who can use voice mail and what they can do with it. ("Dial plans" is actually a term that comes from the telephony world; you'll see some other "phone guy" lingo in the UM documentation that ships with Exchange.) -It answers telephone calls that are directed to its extension (or "pilot number"). When a call is delivered to the pilot number, the UM server uses the extension information provided by the private branch exchange (PBX--or gateway) to look up the user's email address, which the server uses to find the user's mailbox and retrieve the greeting message. Once the caller has left a message, the UM server sends it to the mailbox server. - It provides an Automated Attendant service that answers internal and external phone calls and automates dialing through directory integration with the Global Address List (GAL), acting as a highly advanced switchboard-type application. You can use the Automated Attendant to directly answer all incoming phone calls, or you can restrict its use to callers within your internal telephone system. - It runs Outlook Voice Access, which provides telephone-based access to inbox data using speech or Touch-Tone (dual-tone multifrequency--DTMF) recognition, and offers text-to-speech functionality to read email and calendar information back to the caller. This is an extremely cool feature because it gives you on-demand access to all your email and calendar data from any telephone.

Of course, the UM server isn't the only component involved in making all this magic happen. The PBX that provides your regular switchboard services still handles establishing calls between extensions and accepting inbound calls; when a call to a given extension goes unanswered, the PBX can be instructed to transfer it to the UM server's pilot number, and the magic begins. This implies that your PBX has some way to speak to the Exchange UM server; the means of communication varies according to what kind of PBX you have. Some PBXs directly support the Session Initiation Protocol (SIP), which is used for call setup, and the Real-Time Protocol (RTP), which is used to actually carry voice data. Those PBXs are commonly referred to as IP-PBXs, and they can directly communicate with the Exchange 2007 Unified Messaging role. Other PBXs don't speak SIP and RTP; these PBX systems require the use of a gateway that translates between the native PBX protocols and SIP and RTP. Microsoft doesn't provide these gateways, but third parties do.

When a voice mail message arrives in your Inbox, the real fun begins. Voice mail messages appear as ordinary messages, except that in Outlook 2007 and Microsoft Outlook Web Access (OWA) 2007 they have a special icon. The UM server attempts to resolve the caller's telephone number in Active Directory (AD) to match the number with a name. If you're using Outlook 2007 or OWA 2007, you can play the message on your telephone; with any client, you can open the voice mail attachment and play it back on the computer. Interestingly, you can also get, and play back, voice mails on Windows Mobile devices. Combine that with Messaging and Security Feature Pack (MSFP) push mail, and you have a nifty solution to help you stay in touch when you want to.

In a future column, I'll present a more detailed map of what happens when the phone rings, as well as relating some of my experiences with setting up UM in production. Right now, though, I have to go; my phone is ringing ...