A method and system for operating a telephony service are disclosed. A telephone network is provided, which may be a physical network or a virtual network hosted by an existing network. Network identifies defined by SIM cards, or telephones with built-in SIM cards, are issued to network users. The SIM cards defines a network identity including a unique network telephone number. The network, which is preferably an intelligent network (IN) is configured to permit telephones of the users to receive calls and/or messages, irrespective of the status of a user's account with the network. In addition, the network is configured to permit users without credit or airtime to make an outgoing call to an intended call recipient, the call being disconnected after a predetermined number of rings. This is sufficient to create a missed call notification on a mobile telephone of the intended call recipient. The network is configured to generate a notification such as a text message or recorded announcement to the intended recipient if he/she is not available at the time of the call.