Capacity and power consumption are two main bottlenecks in the wide deployment of Voice-over-IP (VoIP) over IEEE 802.11 wireless LAN, when mobile terminals are used, the impact on the Voice-over-IP (VoIP) by power consumption becomes more critical. In this paper, we analyze the capacity of VoIP traffic in 802.11 WLAN and do the relevant simulations with and without sleep strategy. We find that capacity decreases when the network adopts sleep strategy. After a detailed analysis by p-persistent model of the IEEE 802.11 protocol, we provide a dynamic sleep strategy, namely Collision Detective Dynamic Sleep Strategy (CDDSS) to improve both the capacity and energy saving rate of the VoIP over 802.11 WLANs. In our CDDSS, mobile station (MS) adjusts sleep interval and packetization interval adaptively according to the average collision probability pc of the network. Our simulation results show that CDDSS cannot only improve the capacity of VoIP in 802.11 WLANs, but also reduce considerable energy consumption.