A method of enabling charging information generated within a GPRS access network 4 to be associated with charging information generated within an IP Multimedia Core Network Subsystem, IMSS, the charging information relating to a multimedia call facilitated by the IMSS and carried by the GPRS network. The method comprises sending a SIP call identity from the IMSS to the GPRS network and/or a GPRS charging identity from the GPRS network to the IMSS. A received identifier is included in charging information messages generated by one or both of the GPRS network and the IMSS.