Order the free DID at http://ipcomms.net/ order the free number and the phone number will be e-mail'd to you. You will be asked for the IP address of your box. If you change your IP address you will need to e-mail them with the change.
Go into FreePBX and then Inbound Route. You don't need to add a Trunk. You will only be receiving calls.
Enter the phone number in the DID Number box. Enter something in the Description box to identify the Inbound Route.
Set the Destination of where you want the call to go at the bottom of the page, and click on Submit button.