Quantitative understanding of human communication behavior, one of the fundamental human activities, is of great value in many practical problems, ranging from urban planning to emergency management. Most of the recent studies have focused on human communication under normal situations. Here, we study the temporal patterns of emergency calls, which is a special kind of human communication activity under emergency circumstances, by analyzing a dataset of emergency call records that collected from a metropolitan city in China during a five year period. We find that most individuals rarely make emergency calls. The distribution of inter-call durations decays as double power law along with an exponential tail. We also discover that, comparing with the normal communication activities, the activity of calling the emergency number shows more significant characteristics of burstiness and memory. We further demonstrate that the behavior of calling the emergency number when people encounter extreme events could be explained by an event-driven memory process.