The SIP filter in Microsoft Forefront Threat Management Gateway (TMG) 2010 may prevent an internal client from hanging up a call because TMG may drop the BYE message from the client.
This problem occurs because certain clients may increase the message sequence number (CSeq) for the BYE request if the user tries to hang up the call before the remote end answers the telephone. The SIP filter in TMG does not expect this, and the message is dropped. The call cannot end until the client sends a CANCEL message.
To resolve this problem, install the service pack that is described in the following Microsoft Knowledge Base article:
2555840 Description of Service Pack 2 for Microsoft Forefront Threat Management Gateway 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates
Article ID: 2591278 - Last Review: Oct 31, 2011 - Revision: 1