A module interfaces a mobile communications network to a packet data network and is capable of implementing an Intelligent Network Service for a packet-switched service for subscribers of the mobile communications network. A network interface module may be operative to implement such prepaid charging in real-time. The network interface module may determine both a count and a time period from the prepaid balance, and compare the number of information units exchanged during the service and the duration of the service against the count and time period, respectively. An entry for a subscriber in a Subscriber Information Registry of a home mobile communications network may be defined such that when a subscriber roams into a visited mobile communications network, the network interface module of the home mobile communications network is used to implement a session between the mobile terminal of the subscriber and a node of a packet data network.