You cannot join a Microsoft Office Communications Server 2007 R2 conference by using a telephone when the call is transferred. This issue occurs after you apply the update described in Knowledge Base (KB) article 980370 or in KB article 2028574 on an Office Communications Server 2007 R2 server.
This issue occurs because the conferencing attendant service rejects the INVITE requests when calls are transferred.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Article ID: 2447960 - Last Review: Nov 19, 2010 - Revision: 1