The Unified Messaging features included in Microsoft Exchange Server 2007 allow companies to integrate IP PBXs and unify business communications. The open source IP telephony platform Asterisk is gaining popularity because of its flexibility and low cost. Luckily, it's fairly easy to make the two work together.
If your company is preparing for Exchange 2007 deployment and is considering or running Asterisk open-source IP telephony, there is a way to make the two systems coexist within the same organization.
On his blog, programmer Alan Dutton provides instructions on how to connect Asterisk to Microsoft Exchange 2007 for Unified Messaging, so they'll work side-by-side in the same phone system.
The example documented in the article mentioned above involves running Asterisk as a VMWare image of Trixbox (a Linux distribution with Asterisk pre-installed and pre-configured), and Exchange 2007 with Unified Messaging.
If someone dials in and connects to Asterisk, and then punches 666666, they'll be forwarded to the Exchange 2007 auto-attendant. If they dial 55, they'll be forwarded to the Exchange 2007 Subscription.
The exact number to dial for each is arbitrary, and I believe it can be changed as long as it doesn't conflict with other commands. (I suspect the author's choice of 666666 is a joke.)
According to Alan Dutton, a few things still don't work yet. For one, caller ID is not passed from SIPX to Exchange Server, so voice messages forwarded in this manner are tagged as "anonymous." Additionally, the instructions for dialling out from Exchange Server are included in the linked article, but have not been tested by the author yet.