Put the stuff for the file /etc/fail2ban/jail.conf into /etc/fail2ban/jail.local. According to the howto in your earlier post, you're not supposed to add rules the jail.conf file but jail.local instead.
So in /etc/fail2ban/jail.local you put:
enabled = true
filter = asterisk
action = iptables-allports[name=ASTERISK, protocol=all]
sendmail-whois[name=ASTERISK, dest=root, firstname.lastname@example.org]
logpath = /var/log/asterisk/full
maxretry = 3
bantime = 259200
Do a reload of fail2ban:
# sudo /etc/init.d/fail2ban restart
And now with:
# iptables -v -L
you should see that the asterisk rule is loaded.
Hope this helps,