By default access to UDP port 69, that is used for TFTP, is blocked by iptables (firewall) on SysAdminMan VPS server. This needs opening if you want to use Endpoint Manager to configure your compatible handsets.
When allowing TFTP access we only want to allow fixed IP addresses as there is no security provided by TFTP. You will probably also want to allow NTP (network time protocol) access to allow your Asterisk server to set the correct time on the phone, so we also open port 123.
Run the following commands while logged on as root. Replacing the address (220.127.116.11) with your own public IP address –
# iptables -I INPUT 1 -s 18.104.22.168 -p udp --dport 123 -j ACCEPT
# service iptables save
Saving firewall rules to /etc/sysconfig/iptables: [ OK ]