A method of authenticating a request to change IMS supplementary service data stored at an application server within an IMS network. The method comprises sending from a user equipment to the application server a request for current IMS supplementary service data, on receipt by the user equipment of a response containing the IMS supplementary data in an XML configuration document, and an indication that an access code is required to change the supplementary service data. The method further comprises constructing an amended XML configuration document, incorporating the amended XML document into an Extensible Mark-up Language, XML, Configuration Access Protocol, XCAP, message, incorporating an access code into the XCAP message at a location external to the XML document, and transmitting the XCAP message from the user equipment to the application server.