Here are some basic rules you can use. You can make a file /etc/rc.d/rc.firewall and have /etc/rc.d/rc.local run it so its loaded at boot.
This is pretty basic but better than no security at all, and you can do much much more with iptables.

iptables -A INPUT -s 10.0.0.2 -j ACCEPT
This will allow everything from that address. Specify your own ip or network, so you can ssh to it and specify your sip proxies so it can receive sip packets on the other ports like the backdoor, trunk, etc.

# Flush all Chains.
iptables -F INPUT
iptables -F FORWARD
iptables -F OUTPUT
 
# Set default Policy for each Chain.
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT
 
# Rules
iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT -s 127.0.0.1 -d 127.0.0.1 -j ACCEPT
iptables -A INPUT -s 10.0.0.2 -j ACCEPT
iptables -A INPUT -p udp -m multiport --dports 6000:65534 -j ACCEPT
iptables -A INPUT -p udp -m multiport --dports 5060 -j ACCEPT