The following ports may need to be opened:

Two firewall options are Arno's Firewall or the built-in IPtables.

Arno's Firewall is a light weight and comprehensive firewall based on iptables which is also used in the ASTLinux PBX.

See also Arno's Firewall

See Fail2Ban.

For a base CentOS 6.2 box, after installing Fail2Ban via the EPEL repo, you can just copy and paste the following in one go to get a basic Fail2Ban installation set up for your PBX:

cat << EOF >> /etc/fail2ban/fail2ban.local
# Fail2Ban local configuration file
#
# This file overrides the fail2ban.conf file

[Definition]
logtarget = /var/log/fail2ban.log

EOF

cat << EOF >> /etc/fail2ban/jail.local
# Fail2Ban local configuration file
#
# This file overrides the jail.conf file

[DEFAULT]
ignoreip = 127.0.0.1 209.193.64.0/24 70.176.57.141
bantime  = 600
findtime  = 600
maxretry = 3
backend = auto


[asterisk-iptables]
enabled  = true
filter   = asterisk
action   = iptables-allports[name=SIP, protocol=all]
#           sendmail-whois[name=SIP, dest=none@yourpbx.com, sender=none@yourpbx.com]
logpath  = /var/log/asterisk/fail2ban
maxretry = 5
bantime = 600

[ssh-iptables]
enabled  = true
filter   = sshd
action   = iptables[name=SSH, port=ssh, protocol=tcp]
#           sendmail-whois[name=SSH, dest=none@yourpbx.com, sender=none@yourpbx.com]
logpath  = /var/log/secure
maxretry = 3

[apache-tcpwrapper]
enabled  = true
filter   = apache-auth
action   = iptables-allports[name=PBX-GUI, port=http, protocol=tcp]
#           sendmail-whois[name=PBX-GUI, dest=none@yourpbx.com, sender=none@yourpbx.com]
logpath  = /var/log/httpd/error_log
maxretry = 3

[vsftpd-iptables]
enabled  = true
filter   = vsftpd
action   = iptables[name=FTP, port=ftp, protocol=tcp]
#           sendmail-whois[name=FTP, dest=none@yourpbx.com, sender=none@yourpbx.com]
logpath  = /var/log/vsftpd.log
maxretry = 3
bantime  = 600

[apache-badbots]
enabled  = true
filter   = apache-badbots
action   = iptables-multiport[name=BadBots, port="http,https"]
#           sendmail-whois[name=PBX GUI, dest=none@yourpbx.com, sender=none@yourpbx.com]
logpath  = /var/log/httpd/*access_log
bantime  = 600
maxretry = 1

EOF

cat << EOF >> /etc/fail2ban/filter.d/asterisk.conf
# Fail2Ban configuration file
#
# Asterisk Filter - /etc/fail2ban/filter.d/asterisk.conf

[INCLUDES]

# Read common prefixes. If any customizations available -- read them from
# common.local
#before = common.conf

[Definition]

#_daemon = asterisk

# Option:  failregex
# Notes.:  regex to match the password failures messages in the logfile. The
#          host must be matched by a group named "host". The tag "<HOST>" can
#          be used for standard IP/hostname matching and is only an alias for
#          (?:::f{4,6}:)?(?P<host>\S+)
# Values:  TEXT
#

failregex = Registration from '.*' failed for '<HOST>(:[0-9]{1,5})?' - Wrong password
	    Registration from '.*' failed for '<HOST>(:[0-9]{1,5})?' - No matching peer found
	    Registration from '.*' failed for '<HOST>(:[0-9]{1,5})?' - Device does not match ACL
	    Registration from '.*' failed for '<HOST>(:[0-9]{1,5})?' - Username/auth name mismatch
	    Registration from '.*' failed for '<HOST>(:[0-9]{1,5})?' - Peer is not supposed to register
	    NOTICE.* <HOST> failed to authenticate as '.*'$
	    NOTICE.* .*: No registration for peer '.*' (from <HOST>)
	    NOTICE.* .*: Host <HOST> failed MD5 authentication for '.*' (.*)
	    VERBOSE.* logger.c: -- .*IP/<HOST>-.* Playing 'ss-noservice' (language '.*')

# Option:  ignoreregex
# Notes.:  regex to ignore. If this regex matches, the line is ignored.
# Values:  TEXT
#
ignoreregex =

EOF

service fail2ban restart