/usr/share/a2billing/Cronjobs/a2billing_batch_autodialer.php create a lot of dummy mail to vm@vioblue.com

what is the functions of a2billing_batch_autodialer ?

1. Check on the /etc/crontab
[root@ip-10-142-157-15 cron]# cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

5 * * * * /usr/bin/perl /var/monitortools/checkconf.pl

and there are also schedule jobs within each of those hourly, daily, weekly and monthly. 

Other than the above, you shall also check on the /var/spool/cron

in this directory, your will find those users who has cron job setup e.g asterisk