According to the present invention there is provided a method of charging for a combinational multimedia service making use of circuit switched and packet switched services between participating user terminals and an IP Multimedia Subsystem, IMS, responsible for setting-up and controlling packet switched sessions, the method comprising: sending packet switched data generated by a first of the terminals, from a gateway node of a packet switched access network to the or each other terminal via a routing node having an IP address/port-number known a priori; and configuring a charging function associated with said gateway node to zero-rate data destined for said fixed IP address/port-number.