One of the most sore of sore points for the ISA firewall is the SIP support issue. Those of us with our ear to the railroad track knew back in 1999-2000 that VoIP was going to be the next big thing. It seemed like the ISA team was on top of things, since they included an H.323 Gatekeeper with the ISA 2000 product. It only made sense that as the VoIP scene changed from H.323 to SIP, that the ISA firewall product would keep pace with those changes.
For a number of known and unknown reasons, that didn’t happen. The good news is that there is some light at the end of the tunnel. To find that light, check out Tom Bartlett’s article on ISA, LCS and SIP over at http://blogs.technet.com/accessdenied/archive/2006/02/10/419282.aspx