A gateway (1) resides in an IP-based network. It acts as a controlling or home HLR for a mobile device roaming in this network, emulating an MSC or VLR to the home network HLR. Thus, as far as the home network HLR is aware, the subscriber is roaming in a foreign network of equivalent type of technology. Meanwhile, the gateway of the invention manages a subscriber profile and authentication as if the visited network were the home network. In fact, the gateway in combination with the home network HLR acts as an IMS-standard HSS. The gateway (1) enables wireless carriers to use existing legacy network infrastructure to provide a smooth evolution to the next generation network architecture, and it supports voice and data inter-working between existing 2/2.5G networks and next-generation IP based networks. For inter-standard roaming, the gateway 1 extends the existing roaming footprint of an ANSI-41 subscriber roaming in GSM foreign mode. In GSM foreign mode, the ANSI-41 subscriber may roam onto alternative access networks using a sponsor GSM IMSI.