ISA Team blogged about this error when you are using ISA 2006, OCS Edge and OCS communicator client located outside of your network. The resolution and explanation for this issue are explained in detail on this following address: http://blogs.technet.com/isablog/archive/2009/04/01/ocs.aspx
About The Author
Anderson Patricio is a Canadian MVP in Cloud and Datacenter Management, and Office Server and Services, besides of the Microsoft Award he also holds a Solutions Master (MCSM) in Exchange, CISSP and several other certifications. Anderson contributes to the Microsoft Community with articles, tutorials, blog posts, twitter, forums and book reviews. He is a regular contributor here at Techgenix.com, MSExchange.org, ITPROCentral.com and Anderson Patricio.org (Portuguese).