This specification describes the network feature Operator Determined Barring (ODB). This allows the network operator or service provider to regulate, by means of an exceptional procedure, access by the subscribers to GSM services, by the barring of certain categories of outgoing or incoming calls or of roaming. ODB shall take effect immediately and shall terminate ongoing calls and bar future calls. The purpose of this network feature is to be able to limit the service provider s financial exposure to new subscribers, or to those who have not promptly paid their bills. It may only be applied to the service provider s own subscribers.