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Wireless voice over internet protocol (VoIP) is an important emerging service in telecommunication due to its potential for replacing cell phone communication wherever a wireless local area network (WLAN) is installed. Recent studies, however, suggest that the number of voice calls that can be supported in the widely deployed IEEE 802.11 WLAN is limited. In this paper, we utilize a so-called transmission opportunity (TXOP) parameter of a medium access control (MAC) protocol as a simple solution to improve the VoIP capacity. We provide a detailed analytical model to show that the capacity can significantly be improved and discuss the implications of the TXOP parameter in terms of the maximum number of calls the IEEE 802.11 network can support. The analytical results are validated by simulations for a wide range of parameters. Furthermore, we investigate the impact of the buffer at the access point (AP) on the number of obtainable voice calls. We show that there exists an optimal buffer size where the maximum voice capacity is achieved, but further increasing the buffer beyond this value will not result in an increased voice capacity. Based on this finding, a closed-form expression for the maximum number of voice calls is developed as a function of the TXOP value. Finally, we propose a simple yet accurate approximation for voice-capacity estimation and provide some insights gained from the approximation.