According to a first aspect of the present invention there is provided a method of operating an IP Multimedia Subsystem Application Server to facilitate a communication session between a first user and a second user at a required Quality of Service. The method comprises receiving a notification that a Policy and Charging Rules function associated with the first user has not authorized said required Quality of Service (B4), the notification including an indication of additional Quality of Service required by the first user in order to achieve said required Quality of Service, requesting authorization, from a Policy and Charging Rules function associated with the second user, for the additional Quality of Service for the first user (B11), receiving a notification that the Policy and Charging Rules function associated with the second user has authorized said additional Quality of Service (B12), and notifying the Policy and Charging Rules function associated with the first user that the additional Quality of Service has been authorized for the first user (B14).