Fantastic idea! There are a companies who make VoIP softphones for PDA devices.
SIP softphones work pretty well and kind of act like cell phones, i.e., after the PDA is registered, you dial the number and click “send.” Using MGCP is a little more elegant in by the fact that you can go off-hook and make a call. You could potentially carry your home-phone with you. If the right gateways are in place, imagine traveling anywhere there’s an Internet connection and getting your home-town dialtone!