Well,

First, be aware to have your NT server up-to-date : Service packs, drivers. Then, use standard modems if you can, with the right driver.
Then, there's a Winsock service update at MS site (release 2.x I suppose). if you plan to have multiple connections via a ISDN router, choose a well known brand router (Cisco, 3Com, D-Link).

I think this is a good starting point.

HTH