From what I' ve heard in previous threads concerning this topic the feature is not supported, because ISDN is rarely spread in the world outside Europe. Therefore we schould use external ISDN TAs.
Well, and the link from Rick is nice, but the Howto does not work with the current version of e-smith (4.1.1 if i remember it right)
At least I tried it with v0.4, 2000-20-19 of the HowTo, but no success.
Since I cannot reach the server at the moment I do not know if there is a up to date manual for 4.1.1.
Today I ordered a Elsa Microlink ISDN Internet TA for my E-smith box.
It's rather cheap about 200 DEM (~100 $) and I will post my experiences with it in here.
With kind regards
David Ringhofer