a method and apparatus for allowing isdn devices to communicate over an analog telephone line is shown . a first interface accepts isdn formatted data from an isdn terminal equipment device , removes the d channel data from the isdn data , and provides the d channel data to a synchronous modem . the modem places onto an analog telephone line a modulated carrier corresponding to the d channel data . the modulated carrier is passed through a telephone switch onto another analog telephone line . a second synchronous modem accepts the modulated carrier , demodulates the modulated carrier to recover the data , and provides the data to a second interface . the second interface accepts the data from the modem , inserts the data into the d channel information in isdn data stream , and provides the isdn data stream to an isdn network termination device or isdn switch . the result is that two isdn devices can communicate with each other , even in areas where the local telephone company does not provide isdn capability and the two devices must communicate with each other using a non - isdn medium , such as an analog telephone line . furthermore , an isdn device can be tested and evaluated for compatibility and operation with isdn switch without the necessity of having to transport the isdn device to an area wherein the local telephone company provides isdn switching capabilities .