The High Beams Murder Case

A woman borrows her housekeeper's car and drives off a cliff. She had claimed that her daughter had just called, saying that she was injured at the base of the cliff and needed help. But the daughter insisted she knew nothing of such a call. How could a mother have failed to recognize her own child's voice? What made her lose control of the car? And why did the daughter hire the detectives to prove that the accident was actually murder?