This could be needed for any number of reasons, but I needed to do this to have two trixbox Pro servers live next to each other on the same LAN behind an IPCop firewall. They were reporting back the same IP address to the hybrid hosting source, so inbound connections to both were routing to the one server. Normally this is remedied manually by changing the externip in sip.conf, but that is set automatically with trixbox pro, and not an option.
You need to log into the firewall at the console or via ssh. You need to comment out one line to disable masquerading, and add a few more in its place:
nano /etc/rc.d/rc.firewall
#Individual machine on GREEN
/sbin/iptables -t nat -A POSTROUTING -s 10.0.1.99 -j SNAT --to-source 1.2.3.5
#all other machines on GREEN
/sbin/iptables -t nat -A POSTROUTING -s 10.0.1.0/24 -j SNAT --to-source 1.2.3.4
#all other machines on ORANGE
/sbin/iptables -t nat -A POSTROUTING -s 10.0.2.0/24 -j SNAT --to-source 1.2.3.4
#DISABLE MASQUERADE
# /sbin/iptables -t nat -A REDNAT -o $IFACE -j MASQUERADE
