An interesting (and detailed!) post on the Communications Server Team blog caught my eye a few days back regarding the question of why OCS doesn't support SIP over UDP as one of its transport options.
http://communicationsserverteam.com/archive/2008/05/23/196.aspx
Having previously asked this same question of various members of the Product Group it was nice to see a clarification as to why it isn't supported in the product today.
It also reminded me of a long forgotten project I'd embarked on back in the pre-release days of OCS2007 to get the Enterprise Voice aspects working with SIP trunking via a soft PBX, at the time Asterisk didn't support SIP over TCP/TLS so I was unable to get it directly working, however after much investigation, litres of coffee and a number of soul-crushing dead-end's I was able to get it working with sipX, an open source proxy which has been around for years and which supported SIP/TCP and SIP/UDP acting as the gateway between OCS and Asterisk as the software PBX configured to my SIP trunk and local number on the internet.
A quick Live search today indicates there are a number of different options available to those who (like myself) don't have the luxury of a PBX in their basement and want to try out the telephony capabilities of both Office Communications Server and also the Exchange Unified Messaging functionality in their home lab or virtual environment.
Soft PBX options:
Asterisk, remains a favorite of mine having used it for sometime and it now seems that the next version of the product will include native SIP/TCP Support. Asterisk v1.6.0 is currently in Beta and available for download here: http://www.asterisk.org/downloads
PBXnSIP is another alternative that I've stumbled across which appears to have good support and integration potential for OCS and Exchange and also has the SIP/TLS transport layer baked into it.
Download a trial here: http://www.pbxnsip.com/trial
Wiki outlining how to configure for OCS: http://wiki.pbxnsip.com/index.php/Office_Communications_Server
One area which is still problematic in a pure soft environment is call control functionality. OCS uses uaCSTA as its call control mechanism - neither Asterisk or PBXnSIP support uaCSTA today that I've been able to locate in my quick searches, however as is often the nature of Open Source it will only be a matter of time before someone sits down and writes an add-on with this support.
Over the coming weeks as I get a chance I'll post a series of Step-by-Step configuration posts on having OCS2007 and Asterisk work together in a virtual environment.