UPDATE (Jan.10): Verizon sent a trio of techs today to examine the Digital Lines. They found a splice point on a pole near the Wheeler Farm end of the circuit where water had gotten past the weatherproofing. The splice was re-done, with fresh (and better) weatherproofing sealant applied, and a weather box placed around it.
That's likely the culprit here: the problems started during the snowstorm, so probably water got in there and expanded/contracted repeatedly as the water froze and melted. That expansion wreaks havoc with (relatively) fragile copper telco wiring.
As of 3pm we are back on our main STL. We've noticed the volume levels seemed to have changed somewhat with this repair, too, so we're still tweaking things.