This means that an ISDN adapter with an on-board DSP can also communicate with any device that contains a modem. In the diagram below, you can see that we've added a fax machine.

To perform the job of a modem and a CODEC at the same requires a large amount of processing power. DSPs are very powerful processors. Nevertheless, you need one DSP for each B channel for which you want to use this technique. There is, however, no reason why you couldn't use an ISDN adapter that has a single DSP and a soft modem driver to handle two modem calls at a time.