I too have tried this on a different box, and it works OK, I therefore think the problem maybe related to the Dell OptiPlex 390 System, and needing different drivers ??
Sometimes, CentOS doesn’t have the drivers for a particular system. If it doesn’t, you can spend days trying to find out if someone’s written one for the unit you’re trying to install on, or you can give up and try it on another machine.
As a workaround, you could install VMWare’s ESXi Hypervisor 4.1 or 5 on your Dell and then install the FreePBX Distro as a virtual machine, but that’s not recommended for a production system, and those in the know will chastize me for suggesting it.