Interworking between 3G and wireless local area network (WLAN) is a viable approach to provide some of the services of next generation wireless heterogeneous radio access networks (i.e., 4G). Design of effective joint call admission control (CAC) scheme is crucial to improve call blocking performance of new and handoff calls. In this article, we propose a CAC scheme to handle both voice and data calls based on their load quantity. A data call is given to cellular network only if its data size is below a threshold level and there is enough bandwidth to handle the call. A voice call is preferred to be handled by 3G network as it increases the packet overhead in case of handling with WLAN. We develop an analytical model of a 3G/WLAN hybrid cell for co-existing voice and data services. The model facilitates the study of different co-existing services and their mutual effects. We compute the effect of data call size on the blocking probability and response time of a call in a hybrid cell. © Research India Publications.