Seamless handoffs with Quality of Service (QoS) guarantee play a decisive role in the provision of realtime applications such as Voice over Internet Protocol (VoIP) over IEEE 802.11 networks. During a handoff process, a channel scan needs to be first performed in order to discover new access points (APs), and it can be done by either passively listening to beacon signals or actively exchanging probe messages with new APs. We have previously presented a channel scan algorithm, in which voice activity is taken into consideration and beacon listening is scheduled only during silent periods of a VoIP conversation. To further improve handoff performance, a hybrid channel scan mechanism is proposed in this paper, wherein beacon listening is first performed in a handoff, and, after reaching a switching time running estimated during the beacon listenings, probe exchanging can be incorporated to enable the success of the handoff. Extensive simulation results show that the proposed mechanism effectively reduces both packet loss rate and handoff failure probability.