The solution was to add an allow rule to IPTables for port 4445 ...
This has not been a permanent solution because it keeps getting overwritten.
It is, because all settings are generated from within FreePBX firewall module.
The FreePBX interface is not that obvious, but the firewall's services are configurable if you go to Connectivity -> Firewall. Then the tab at the right edge of the screen, then Services -> Custom Services -> Create New Service -> TCP/4445
