In beyond 3G networks the user will not be aware of the access network technology used to provide a telecommunications service. Heterogeneous network technologies will be seamlessly integrated in one "common" access network, enabling users to move around and continuously receive their subscribed services. In a commercial environment, this network evolution requires that a telecommunications operator jointly manages its networks resources to improve the service offered to the users and, at the same time, to increase its revenue. Starting from the UMTS and WLAN interconnection architecture defined by 3GPP, this paper analyzes the performance of a new joint radio resource management strategy, comparing it with two well-known strategies used in scenarios where both networks, the UMTS and the WLAN, are interconnected. The new strategy presented in the paper bases its decisions on criteria related to user mobility characteristics and the application characteristics. The strategy also introduces the possibility of renegotiating new calls and reallocating running calls from one access network to another. The performance analysis considers two traffic scenarios. One where only real-time applications are running and other which also introduces TCP applications. The comparison studies show the proposed strategy outperforms the other strategies in what concerns call blocking probability and applications QoS support. Besides, the proposed strategy tends to reduce the handoffs between networks.