The Session Initiation Protocol (SIP) is a text based signaling protocol, which has been adopted in Next Generation Network (NGN) for controlling multimedia communications. The present paper addresses issues related to Call Admission Control (CAC) for a private network accessing the rest of the world through various links. The private network architecture blends various devices/components including a Call Server (CS) and a Session Border Controller (SBC) . The standard SIP messages are used to share and distribute required information between different devices in the network. In this architecture, an efficient CAC function should take account of many factors, including user profile, call type and the state of the available access links. We propose to distribute the CAC function between the CS and SBC; the CS handles issues related to user profile and the SBC handles issues related to resource availability. The collaboration between CS and SBC is made possible by using the ”SIP Priority header” to share and exchange information during Call Setup. This mechanism helps in isolating the service, control and transfer planes from each other.