In this fic, Blaine's mother was abusive. His father has no idea, and Blaine is terrified when his dad leaves for a business trip (or something like that). The abuse only gets worse now that his father isn't around. During one of his mother's outbursts, Blaine calls Kurt. If I recall correctly, the phonecall was either made accidentally or he had started to call just before his mother started raging and he didn't hang up. Either way, Santana is the one that answers because Kurt's in the shower, and she's her usual snarky self until she hears the abuse happening on the other end. When she realizes what's happening she gets Kurt.
I don't remember exactly what happens after this point, but I do remember Blaine's father feeling like crap because he had no idea his wife had been abusing their son.