In this paper, we present the experimental measurements of real-time voice packets over a 3G-WLAN interworking system. It is of interest to evaluate: (i) the performance of VoIP with an IPSec tunnel in a 3G-WLAN interworking system; (ii) maximum number of VoIP connections in a single 802.11b access point (AP); (iii) handoff latency due to terminal mobility; and (iv) impact of terminal mobility on the VoIP performance. Our results show that the performance can degrade significantly with an increase of VoIP connections and with the terminal mobility. Furthermore, the performance degradation affects all the existing VoIP connections, not merely the new connection or the user equipment which undergoes mobility. The amount of delay during terminal movement severely affects the performance of all realtime applications in that particular AP.