I had to open a port to utilize a merchant services gateway, FirstData. If you want a REALLY easy way to do this, without SSHing or doing anything technical, follow the steps here.

Log Into WHM as root, or a user with elevated access, and head to ConfigServer Security&Firewall. From there, click Edit Firewall Configuration. Add your port to the end of the Incoming/Outgoing TCP Ports List and reset lcf. Your good to go!

Have fun.