When you access an external Session Initiation Protocol (SIP) server through Microsoft Forefront Threat Management Gateway (TMG) 2010, the SIP client may be unable to register with the external SIP server.
The TMG SIP filter will incorrectly drop responses to a REGISTER request if the response contains multiple contact headers. This prevents the SIP client from being able to register with the external SIP server.
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.
To work around this issue, you can disable the TMG SIP filter. To do this, in the TMG Management Console, under Array, under System, click Application Filters, and then click to clear the TMG SIP filter check box.
Note To disable the SIP filter on an Enterprise joined server, you must update the Enterprise configuration under Enterprise Add-ins. To do this, click to clear the Enable this filter on all arrays check box.
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