This work details a simulation-based study of the performance of an IEEE 802.11e Medium Access Control Layer (MAC) over an IEEE 802.11g Physical Layer (PHY). Both the Direct Sequence Spread Spectrum Orthogonal Frequency Division Modulation (DSSS-OFDM) and the Extended Rate PHY Orthogonal Frequency Division Modulation (ERP-OFDM) PHY modes are considered. This study focuses on the number of simultaneous bidirectional G.711 Voice Over IP (VoIP) calls that can be supported by such a Wireless Local Area Network (WLAN). The results show that the DSSS-OFDM mode introduces a very significant overhead for the sake of backward compatibility with 802.11b. In fact, the DSSS-OFDM call capacity is limited to 12 VoIP calls when using the 24Mb/s data rate and 13 when using either the 36Mb/s or 54Mb/s rates. These results demonstrate the well-known uplink/downlink performance disparity. In the cases studied, it is the downlink performance that limits the number of calls that can be carried by the system. The results also show that when a significant amount of lower priority traffic is introduced into the system, it can have a notable impact on VoIP call capacity despite the use of 802.11e. The ERP-OFDM mode is shown to have a much higher call capacity, approximately 42 calls. Above this capacity, loss and delay levels increase to such an extent that the Quality of Service (QoS) of a VoIP call is badly compromised. Two notable characteristics of the heavily loaded system were observed: firstly, an increased number of collisions and retransmitted packets; and secondly, a disparity between the performance of the uplink and downlink transmissions. A new scheme for adapting the parameters is proposed, based on these observations. The new scheme dynamically adapts the Contention Window (CW) based on the retransmission rate of the system. In addition, an adaptive Transmission Opportunity (TXOP) mechanism aids in balancing the uplink and downlink traffic levels and so provides the uplink/downlink performance equality that is required for bidirectional VoIP traffic. The proposed scheme can thus maintain acceptable levels of QoS for higher call capacities, increasing the overall VoIP capacity of the system.