If I understand your question correctly you are interested in ICS with native NT technology. You mention no specific problem with the ISDN DUN from the server. I will assume that ISDN DUN works correctly from the server, but not (as you stated) frominside clients.
I don't believe that you can accomplish ICS with native NT 4.0 even with RRAS (of this point, I am not certain). I know that Proxy server is designed to accomplish this task as an add-on server to NT. An added benefit is the firewall and web-cache features without the licensing headaches of other MS products.
I do know that ICS is built into both Windows 2000 professional and server. In Win2k pro it is accomplished through a share tab on the properties of the ISP connection object (I use this on my home network). In Win2k server it is accomplished through RRAS setup (according to my 70-215 Study Guide).
Proxy is a pain to setup and configure but it is well worth the trouble. In my last job the MIS bought a monitored hardware/software firewall solution from a well-known Intl. vendor for $100K. They recommended an additional installation of a MS proxy server for web-cacheing alone!
Hope this helps, Ray