VoIP capacity is an important metric as it determines the maximum number of calls that can be supported by a Wireless Local Area Network (WLAN) before call quality degrades. To this end, researchers have conducted extensive simulation and analytical studies to determine the VoIP capacity of different WLANs. These previous works, however, assume stations are always awake during a call. In 2005, the Wi-Fi Alliance proposed a power saving mode extension that allows stations to retrieve packets from the Access Point (AP) at any time. In light of this development, this paper derives the VoIP capacity of a IEEE 802.11a WLAN where stations sleep for different time intervals. Moreover, it proposes a novel opportunistic scheduler that addresses a critical problem that arises when the power save extension is used in conjunction with a solution that improves the VoIP capacity of a WLAN by aggregating packets.