I’ve discussed Microsoft Exchange Server 2007’s new unified messaging (UM) feature and the PBX systems it supports. Specifically, I wrote that you can’t use Exchange 2007 UM with the popular Linux-based Asterisk PBX software because Asterisk runs the Session Initiation Protocol (SIP) over UDP, whereas Exchange 2007 UM and most other commercial VoIP products use SIP over TCP. I noted that you “might be able to cobble something together” using a gateway solution but that I didn’t know about a supported way to make the connection.

In response, a reader emailed me about taking a similar idea in a different direction: trying to get Asterisk to work with Live Communications Server (LCS) 2005 and succeeding by using an open-source tool called SIP Express Router. SIP Express Router converts SIP packets between TCP and UDP so that Asterisk and Microsoft’s products can interoperate. If you’re not already an Asterisk or SIP guru, you might find the full configuration instructions inscrutable, but they’re worth looking at to get a feel for what you need to do to get it set up (http://www.voip-info.org/wiki/view/MS+LCS+2005+%252F+SER+%252F+Asterisk+Integration).

What is the value of integrating a PBX with LCS 2005? First, LCS 2005 is available now; you can deploy it without waiting for Exchange 2007. Second, LCS 2005’s feature set complements Exchange nicely; Exchange provides voicemail integration and a telephone UI for Exchange calendar, contact, and email data (plus faxes and the Global Address List—GAL), and LCS provides presence, IM (including audio and video), and call control. LCS’s call control features require the software to be able to talk to the PBX so that the software can properly route incoming calls.

How do LCS 2005 and its successor, Office Communications Server (OCS) 2007, work with Exchange UM? Let’s say that Alice is calling Bob and that Bob’s Active Directory (AD) account is provisioned for both a UM-enabled Exchange 2007 mailbox and LCS. When Alice dials Bob’s number, the following things can happen: 1. If Bob is talking on the phone, the PBX automatically routes Alice’s call to the Exchange UM server. The UM server records a voicemail message, which appears in Bob’s Inbox. 2. If Bob is in the office or working outside the office while connected to the office through a VPN when Alice calls his office phone, LCS sends a call notification to Microsoft Office Communicator 2005, running on his computer. This notification causes Communicator to generate a pop-up message on Bob’s computer screen using the familiar “toast” window style (like MSN Messenger and Outlook). Bob can use the toast window to take the call on his computer or transfer it elsewhere (e.g., a mobile phone). If he transfers the call, LCS instructs the PBX to perform the transfer. 3. If Bob doesn’t want to take Alice’s call, the phone will keep ringing.

Depending on how the PBX is configured, the unanswered call will eventually be transferred to the Exchange UM server, letting the caller can leave a voicemail message. One benefit of using Asterisk to enable these scenarios is that it’s inexpensive compared with traditional PBX systems. Even if you buy a preloaded server running Asterisk (and many companies provide such complete, turnkey products), the cost is significantly less than buying a new IP PBX, which especially helps small-to-midsized businesses that want to deploy UM, enhanced presence, and telephony integration. Of course, the proof of whether Asterisk is a workable alternative will come when we see it working with Exchange 2007. Stay tuned!