A media-independent access control function is proposed to arbitrate access of two types of traffic running on a terminal with concurrent accessibility to two different networks. Specifically, voice and data traffic, and UMTS and WLAN networks are to be considered. To improve its QoS, voice traffic is given preferential access to the Cellular network, while data is assumed to have preferential access to WLAN network. To further improve the QoS, the two networks are assumed to serve voice traffic with higher priority over data. Through simulation, the effects of the two sub-functions of the proposed scheme on the system performance will be investigated.