I have an idea of a voip gateway box. I guess you would call it that. I don't think this device exist yet.
You could call from your cell phone or another local number to your voip line. It would pickup and give you a fake ring to enter an access code. After the correct access code, you would get get a simulated dial tone or beep. Then you would enter the long distance number you want to call. The voip line would then use three-way calling to call the number you just entered. When your done with the connected call you could punch a special tone sequency to disconnect all connections.
This would be a way to make a free long distance call from a cell maybe that you can make free local calls on. Also, when you fist call in and don't enter the special security access code to use the device after so many seconds. The device would trigger the other house phones to ring like normal. Maybe this could be done with a modem and software?