Hi morsepi look on /etc/asterisk/jail.conf under section [asterisk] the parameter logpath = /var/log/asterisk/full, should be pointing to the log that is registering the failed SIP registration atempts, after editing issue the command, according to your CentOS version:
[root@uc ~]# systemctl reload fail2ban
[root@uc ~]# /etc/init.d/fail2ban reload
In my case, the file originally was pointing to messages log file, and this log was empty, as soon as I change the logpath parameter, and reload fail2ban it start to ban.
Hope this helps.
David
