Carriers are notorius for pointing the finger at the vendor and the customers' phone system even when they know the problem is on their end. If you're confident that your equipment and your programming is all good, see if you can get one of their switch techs on the phone with you and then give him the run down on All of your related settings.
Are they saying they can't get the D channel up or are they saying the Norstar is bringing it down?
You are using at least 6.1 software, release 3 or higher right???
Phonehed in Dallas