Voice over Internet Protocol (VoIP) is becoming popular to end-users and sparking great interests in broadband wireless networks. As a mobile version of Worldwide Interoperability for Microwave Access (WiMAX), IEEE 802.16e plays an important role in the evolution towards 4G. In this paper, we have investigated the actual quality of VoIP in the 4G field trial, which aims to better understand the mobile WiMAX performance for delay-sensitive services. Distinct from the past research works, we set up two different evaluation systems, professional and user-friendly. The performance measurements utilize Perceptual Evaluation of Speech Quality (PESQ) to evaluate the voice quality, the packet loss, jitter and delay for network tests. Our test results show that the mobile WiMAX network is able to support well the delay-sensitive VoIP service. We find that the VoIP quality on the downlink is perfect. On the uplink, the quality is degraded but still adequate at both cases of the cell edge and handover. Our work makes a contribution in better understanding the mobile WiMAX performance for both industry and academia