If you disconnect all the telephone wires from where your regular phone wire from the phone company comes into your house/apt or whatever, and then connect a regular phone (RJ11) phone cord from any one of the jacks in the house to the cable modem, and then use a doubler (a thingy you can buy from any best buy circuit city radio shack etc) you can plug the cord from any of your phones into any of the jacks, and all the jacks will be connected to the modem.  DO NOT DO THIS UNTIL YOU HAVE DISCONNECTED THE "INSIDE WIRING" FROM THE WIRE COMING FROM THE PHONE COMPANY . . otherwise the "ring voltage, which is about 90 volts, will FRY your modem . . a very bad deal.  FWIW I use Vonage, in the same setup with relatively poor audio vs the old "landline" telco, but it is a LOT cheaper.