VoIP is a rapidly growing technology that enables the transport of voice over data networks such as the public Internet. VoIP is becoming a viable alternative to the public switched telephone networks (PSTNs). In parallel, a dramatic increase is happening in the deployment of Wireless Local Areas Networks (WLAN) in buildings and corporate campuses. WLAN is, nowadays, mostly used for ordinary data services such as web browsing, file transfer and electronic mail. However, with the emerging usage VoIP telephony, WLAN are sought to be used as access infrastructure for enabling such applications. One of the issues of using VoIP over WLAN is the effects caused by users roaming within and between WLAN subnets during a VoIP session. The latency and the jitter are greatly impacted when the control of the mobile node is handed over from one access point (AP) over to another one. This poses a challenge to providing and preserving QoS for VoIP users in WLAN environments. In this paper, we propose to study and measure the effect of the hand-over for both intra and inter mobility for VoIP traffic.