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.
To fix this problem, install the following cumulative update:
2403680 Description of the cumulative update for Office Communications Server 2007 R2, Conferencing Attendant: November 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.