I would welcome then, verify them, and listen (and take notes) as I let them vent. I would personalize the call by using their name at least twice. I'd tell them what I'd heard them say as I'd try to imagine it happening to myself. I would offer a sincere apology, and tell them how I could help and proceed to do so. I'd explain it all back carefully, empathize, offer one more apology, and make sure they were reasonably satisfied before closing the call and coding it.