Here is a simple to-do list of what you'll need to do to set up Free PBX with multiple phone lines in a small-office style configuration. Free PBX is part of the Plug PBX system.
Adjust your settings
When you visit the IP address of your Plug PBX you are presented with a menu page that allows access to various parts of the PBX system. The link to 'FreePBX Administration' will send you to the main administration screen.