A wireless intranet office (WIO) concept is disclosed, which integrates an IP based Intranet environment (27) and GSM network providing mobile telephones (21) with access to GSM through the GSM network or via the intranet. Access through the intranet to the GSM MSC (26) is provided by a WIO interworking unit (24) which may comprise several network entities (e.g. intranet mobile cluster (241), intranet location register (242), WIO gatekeeper (243), WIO gateway (244) and H.323 gateway). A dual mode terminal for such a system is also disclosed.