With the widespread use of wireless sensor networks (WSN), more types of applications are emerging. However, controlling a deployed wireless sensor network after deployment still requires expertise on embedded system programming and administration. In this paper, we study how to exploit the widely available smartphones to reduce these restrictions on users' expertise. In particular, we present a system called PhoneCon, which stands for Voice-driven SmartPhone Controllable Wireless Sensor Networks. Through PhoneCon, users may simply speak to their phones to operate a deployed wireless sensor network, such as getting the status of deployed motes, without being exposed to any of system or programming level details. We have implemented PhoneCon on Android phones for a testbed of MicaZ sensor nodes. Our evaluation results show that PhoneCon can interpret users' intentions well enough through the microphone and perform corresponding actions with acceptable delays.