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http://forums.isaserver.org/ultimatebb.cgi?ubb=get_topic;f=18;t=000165
But note, what microsoft says:
http://www.microsoft.com/technet/prodtechnol/isa/2000/maintain/isaimsec.mspx
SIPS. Features such as voice, video, application sharing, and whiteboard require a connection to be made between an internal and external client, and use SIP Signaling (SIPS) to set up the communication session, which then uses dynamic ports. For example, using audio/video (AV) requires opening all UDP ports between 5004 and 65535 to allow SIP and media streams (RTP) to cross the firewall. The use of dynamic ports without an associated application filter is a problem because ISA Server does not have information about which ports to open and at what time. No ISA Server SIP application filter is available to circumvent this issue.
and another one:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rtcclnt/rtc/traversal_of_upnp_enabled_nats.asp