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Here is a hint of how the latter might be accomplished- http://list.sipfoundry.org/archive/sipx-dev/msg27891.html

Here is an example of adding a custom rule to the running config. I'm not sure whether this will survive if sipXecs decides to rewrite the firewall configuration:

# allows any traffic from a certain subnet
iptables -A INPUT -s 10.1.2.3/24 -p tcp -j ACCEPT
/etc/init.d/iptables save

Disabling the Firewall (Redhat-based systems)

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