It's tricky to diagnose.
I'm sorry, but that just doesn't make sense. The FIRST thing Sonic should do is tell the DSL customer something like:
"This is rare, but it happens. So the first thing you should do is replace your telephone cord, because sometimes that's all that needs to be done."
Look at all the time and work wasted because that simple instruction was never given! My impression is that since DSL is going the way of the dailup modem, technicians are losing basic knowledge around DSL. Furthermore, I'd say that a sudden drop of both Internet and phone is a sign that, yes, replace the phone line and see what happens next. For that WOULD be the "most likely suspect." Sounds to me like you're just making excuses. What, in my case, after hearing my original complaint, would YOU say is the most likely suspect?
I have a hunch there is NO need to send a technician out to my place again, including AT&T. I'm guessing there IS no more of my line being "a lot slower and all over the place," as Jacob stated early in this thread, now that I have installed a new phone line. Shouldn't that be checked now, before going through the motions of another visit? I mean, you'd THINK that, after I reported everything's working fine again, someone at Sonic would do another remote check, to see if everything's all cleared up. So why hasn't that happened? It would save your company both money and time.