If your built-in firewall is active, it may prevent you from using LoudHush. This document describes how to configure the OS X firewall to permit IAX traffic.
Bring up the firewall configuration panel from System Preferences:
Add a new rule that defines the port number for IAX traffic (UDP port 4569):
As long as you leave the IAX rule active, you should be able to use LoudHush while the OS X firewall is active.