The best thing to do is buy a low end VoIP gateway with an FXO port on it. Check out http://www.miptel.com and look at the Combox I personally use this hardware. Comes with 1 FXS port and 1 FXO port, but only enough resources to run 1 at any given time. The call would come in on the FXO port from the phone company and then you will setup the routing to hit you on the assigned IP phone while traveling.
Please let me know if I can assist you any further