If AT&T won't bother to fix the problem, you can file a Public Utilities Complaint. They hate those, it is more paperwork than they want to deal with.
From all of this detective work, I would say we have determined the 7100 to be inadequate at handling line noise that is commonly handled by older Samsung equipment and if indeed Samsung does care about it they won't tell you if they are working on a cure or not so your only two choices as I see it are to give them a refund or have AT&T do their job.
