Most corporate networks will block access to the port used by cPanel. Additionally, some firewalls that are pre-configured on broadband wireless equipment will also block this port. For example the Virgin Media firewall setting will currently have to be set to "Low" to allow access to this port.
Standard internet protocol uses port 80 which is allowed through the firewalls on nearly every corporate network and certainly on home routers or access equipment.
However, cPanel uses port 2083 which is the same as the Secure RADIUS service, encrypting information passing between a client and the server. Many firewalls will be set to block acess to this port, along with many other ports that are not commonly or widely used.