The increasing deployment of Universal Mobile Telecommunication System (UMTS) networks and WLAN (Wireless Local Area Network) hot-spots will shape the future development of wireless and mobile communication systems. The key objective of systems beyond 3G [4] besides the integration of existing systems to interwork with each other and with new air interfaces is an advanced mobility management across technological and administrative domains. Mobility management is seen as an enabler to perform accounting, charging, and billing of mobile users in a multiple Mobile Network Operator (MNO) environment.