Hi,
I'm using a Hayes Accura 15700 external TA and can't fault it. I've used it on both BT and NTL lines. If you only want to use a single ISDN channel you don't have to change a thing. Just plug it in and go! The Accura is smart enough to treat ATDT instructions as straight-forward ATD instructions. If you require to use more than one channel (depends on your ISP but most support 128K ISDN now) then a quick edit of a couple of files will sort it out. There's lot's of detail on this elsewhere in the forum, but if you're really interested let me know and I'll dig out my install notes and post the details here. All in all a great sub-£100 product that's given me no problems so far.
Richard
I just remembered one thing though. I had to use a win95 machine to initially configure bandwidth on demand etc, but the utility supplied with the TA kept crashing my machine. Hayes e-mailed me a beta copy of a later utility that worked fine. I can drop you a copy if you want it.