Call detail records are generated for usage of a telecommunications service established in a telecommunications network, by monitoring messages in a signalling system for co-ordinating operation of the network and carried over signalling links in the signalling system between a service switching function entity and a service control function entity. Messages are selected that relate to instances of usage of the telecommunications service initiated by the service switching function entity to at least one signal switching point in the network. These selected messages are correlated in accordance with the usage instance to which they relate, and information in the correlated selected messages is used to derive a call detail record for an instance of usage of the telecommunications service.