Method and apparatus relating to online charging in an IP multimedia subsystem

Online charging within an IP Multimedia Subsystem (IMS). A method comprises, at an IMS charging node: (a) receiving a credit control request message from an IMS service network node over a service charging interface provided between the IMS service node and the IMS charging node; (b) determining with reference to one or more triggering conditions that an announcement is to be provided to a user associated with the credit control request message and/or another user; and (c) following such a determination, initiating an announcement service in the IMS service node by sending an announcement request in a credit control answer message over the service charging interface to the IMS service node, the credit control answer message being in response to the credit control request message and the announcement request comprising announcement information relating to the announcement to be provided.