Next Generation Wireless Networks (NGWNs) are expected to provide high data rate and optimized quality of service to multimedia and real-time applications over the Internet Protocol (IP) networks. To achieve these goals, handover plays a very critical role in maintaining the seamless connectivity when mobile terminals move across different cells or networks. In this paper, we propose a novel scheme compliant with the IEEE 802.21 standard for handover in an integrated scenario with UMTS and WiMAX networks. We use the call quality, measured using Mean Opinion Score (MOS), as the major metric for handover optimization. We compare the proposed MOS-based handover scheme with the traditional RSS-based handover scheme. The numerical results demonstrate that our proposed scheme can maintain high call quality and reduce the probabilities for both handover dropping and call dropping.