A method of reserving credit for a mobile subscriber in respect of an IP multimedia service. The method comprises, following initial registration of the subscriber to the IP Multimedia service but prior to invocation of an IP Multimedia service, using the early session establishment procedure to reserve a credit amount at a charging control node and to notify an IP Multimedia Serving Element of the credit reservation, whereupon, at invocation of said IP multimedia service, the IP Multimedia Serving Element is able to proceed immediately with the session establishment.