Yes I am referring to Asterisk (www.asterisk.org), Moya.
You can use Asterisk as your “NAT/firewall which will allow voice traffic through.”
I have a 667 mhz system with 128 MB of ram and a 10 GB drive running as my PROXY/FIREWALL/PBX System with Linux and I have 2 XP systems behind that connected to a hub along with my IP Phones and softphones. When a call comes in on the FXO card (we have them for $75 ea) it rings both my IP Phone and my Cell phone through NuFone at 2.9 cents / minute out my cable modem. So wherever I am I can receive calls. It only costs me money when I answer my cell phone. If I dont answer either of my phones you can leave vmail which I can later retreive by dialing the voicemail application or by checking my email.
I can also drop people in a conference room with my ip phone and go out to my car , dial into my fxo and enter the conference number to pick up the conversation where I left off.