Voip security

Working on a recent pen test I ran across a system which was a Cisco
Voip gateway allowing SIP and H.323 protocols externally.  Only those
two ports were available.

1)  I fired up Sivus to scan the SIP protocol and found that it
allowed unauthenticated invite messages into the server.  I follow
this up with further research utilizing netcat to receive messages.
After the initial "200 trying" response I received a couple of "403
forbidden" responses.  My understanding is that the 200 trying meant
that the gateway passed the message back to the back end server and
that server gave the forbidden.  Yet Sivus flagged the vulnerability
as High.  Am I right in assuming if I knew more about the layout I
could try such attacks as a denial of service by ringing all of their
phones at the same time, or sending SPIT?  What potential flaws could
come from this?

2)  They also had the H.323 protocol available, but SiVUS doesn't
support scanning of that protocol yet.  Anyone know of tools,
methodologies to test this protocol?

Thanks Guys,
Mike Klingler

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