Let me bottomline this. The TAPI devices I've seen were very finicky as to the OS it ran on. My bet is they haven't run it on XP yet. Or ported it.

Dell and Microsoft are not who you need to deal with. If it's a Nortel "device" then you have to deal with them and follow their setup to the letter. If it means Windows NT 4.0, with rutaba stiping, then that is what it is.

I'd love to comment more (having done TAPI), but my experience is that it's a tightrope (or straightjacket?) setup. And Microsoft/Dell is not the place to call.