That's one thing with inanimate objects like telephone, they don't have any sense of their own, they don't see what's happening around. Theirs is to respond to what's already programmed.

If only the phone knew what was at stake, it could've blocked Kalu's voice.

On the other side, Kalu seemed to lack manners. Why would he raise his voice when he saw his sister was on phone. Well, I think, things just wanted to get bad.