see #161773 3CX15.5 on Debian activation wizard uses google dns 220.127.116.11 to resolve the needed 3cx hostnames. in closed enviroments with deny-all firewall rules this is not going to work. also this is not mentioned anywhere that outgoing dns on port 53 is needed for license activation. instead the activation wizard should use the dns server from the local interfaces configured dns servers. or maybe should try first google dns server and as backup dns server the ones /etc/resolv.conf. in this way this should work also in deny-all enviroments.