Roast her back, say things like this: "Well, then, as a mother, you're not doing your job properly are you?" and "Mum, how old are you? Do you really think it's appropriate to talk to your own child like that?" and "Where did you get your hair done? At a pet shop?" and "Okay, why don't you just stand in the middle of the town road and make some money with your clothes?" or just hang up immediately after she insults you, then she may know that she's gone too far.