I run a check_mk deployment and try to add everything to it. I recently deployed 3CX for a relative and wanted to include this on my check_mk monitoring.

• SSH to the 3CX host, either as root or sudo -i after login
• Execute the following commands
• wget https://check_mk/agent/url/from/your/check_mk/deployment
• dpkg -i check-mk-agent_2.0.0p17-1_all.deb
• systemctl enable check_mk-async
• systemctl start check_mk-async
• vi /etc/nftables.conf
• Add the following firewall rule, substituting 192.168.0.1 with the IP address of your check_mk master

table inet filter {
    chain input {
[..]
        # check_mk
        ip saddr { 192.168.0.1 } tcp dport { 6556 } counter accept comment "check_mk agent"
[..]
    }
}

• Save the file with :wq!
• reboot
• Login to check_mk and add a new host with the 3CX supplied FQDN