Abstract: A roaming gateway has at least one IP network interface, and at least one cellular network interface, and inter-working functions for inter-working between a wireless IP network and a cellular network. The inter-working functions communicate with user devices in an IP network according to a machine-level subscribe-and-notify event package protocol for managing handover of mobile devices roaming between IP and cellular networks. They also poll a cellular network for subscriber status on that cellular network in the absence of a mobility event package. The inter-working functions comprise HSS functionality programmed to access data on a HLR of a cellular network and to perform HSS operations using said data. The gateway applies routing policies specific to call types, allowing or disallowing the anchoring of calls originating in either an IP network or a cellular network based on number prefixes. The inter-working functions comprise an application server programmed to communicate with a user device as a client.