The widespread of a Wireless Local Area Network (WLAN) access network in residential areas provides low-cost and high-speed access to the Third-Generation Partnership Project (3GPP) services for end users. This has virtually made WLAN to be a complementary access network to Universal Mobile Telecommunications System (UMTS). The interoperability of WLAN and UMTS is made possible with the use of a mobility management mechanism in IP Multimedia Subsystems (IMS) architecture. Current problems in mobility management mechanisms are found in device mobility such as mobile phones. Such problems affect several layers and disrupt services. In this paper, IMS mobility management mechanisms with G1 mobile phone as an IMS client testbed is implemented, and its impact on the Quality of Experience (QoE) during end-to-end VoIP call session and its conformity to the 3GPP requirements of seamless service continuity is evaluated. The QoE driven mobility management mechanism is then proposed and evaluated to improve the QoE and to meet the 3GPP requirements of seamless service continuity during the handover between UMTS and WLAN access networks. Preliminary results show that there is an improvement in QoE and the requirements of 3GPP service continuity were met. The proposed Quality of Service (QoS) driven mobility management mechanism will help in the development of dual-mode mobile handsets with QoE adaptability.