iPhone combusts after 40-minute call

Whatever the details, it is sad to see that a woman in China has been injured when her iPhone combusted after she had been talking for 40 minutes. Having been subjected to 40-minute-long one-sided phone conversations by other passengers on buses and trains and, even, by fellow diners in restaurants, I have some sympathy with the concept of spontaneous combustion. My calls take no more than 40 seconds; anything longer and I write an email.