Few info about Cron needed

In other linux systems I usually edit /etc/crontab to execute periodically a wanted script/process
It seems not work in FreePBX distro

I used crontab -e to edit table/add processes and crontab -l to display scheduled operations: is this the right way to proceed ??

I have a distro with sysadmin pro module , system update has no interval checked thus , looking at /var/log/cron, /usr/sbin/sysadmin_update_system -a is executed every second or less, I set it to none and crontab entry has gone.
Is this a bug ? Is it enough to disable system update ?

Thank you