If the system needs to be set to listen on multiple SIP port (e.g. 5060 and 5061):

Log in to the system shell as a root user.

If you're using Centos, Fedora, or Red Hat, install the iptables

        yum install iptables

If you're using Gentoo, install the iptables

        emerge -v iptables

Add the new SIP port (5061) by executing:

iptables -t nat -A PREROUTING -p udp -d $YOURPUBLICIPADDRESS --dport 5061 -j DNAT --to $YOURPUBLICIPADDRESS:5060

where $YOURPUBLICIPADDRESS is the actual IP address of your server e.g. 80.65.65.221)