I've installed 3CX on a new Debian server from OVH following the instructions on this site. I've not used Debian before. What do I need to do to firewall it off. The 3CX firewall checker shows all the ports required are open - but does that mean there is no IPTables running? What do I need to do to secure the system before I connect my SIP trunks. What is best practice? Thank you!