When we need to setup firewall for Asterisk, you should know that you need to have many opened ports for himself. At end of your file /etc/sysconfig/iptables you should add this:

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 10000:20000 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 10000:20000 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 4500:5100 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 4500:5100 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 4569 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 5036 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 5060 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 5060 -j ACCEPT
COMMIT

-A RH-Firewall-1-INPUT is from default Red Hat firewall. You can change this to name you want. Also you should know on which port's your VoIP client should listen so you need open and that ports. After this restart iptables.

[root@asterisk1 ~]# /etc/init.d/iptables restart