The specification of the SIP UNI in this document is based on a standard SIP User Agent (UA) connected to a Service Provider Network composed of standard SIP components (SIP proxies@ etc.). Depending on the service scenario@ the SIP UA can interact directly with an Application Server@ or with another SIP UA via a SIP Application Server. The SIP UA implements standard SIP session control mechanisms for initiating sessions@ and for responding to invites from another SIP UA or Application Server. Services are implemented in the SIP UA@ in the Application Server (with the SIP UA interacting with the Application Server through standard SIP mechanisms) or in a combination of the SIP UA and Application Server. This specification applies to a SIP UA associated with an individual SIP device as well as to a SIP UA associated with a SIP PBX. From the perspective of this specification@ a SIP UA connects to a service provider network either directly@ or via a SIP PBX .