With the telephony business world focusing on Voice over IP (VoIP), numerous IP Private Branch Exchange (PBX) vendors are offering rich new product lines supporting VoIP capabilities, presence integration, and other enhanced multimedia and location-based services. In order to achieve true convergence of these technologies, PBX, phone and network vendors will need to design and support products that will interoperate with each other based upon industry standards. Session Initiation Protocol (SIP) is being widely adopted in the industry [1] as a signaling protocol because it enables data and voice convergence for devices and applications across a wide range of industry sectors. SIP enables voice, video, Instant Messaging (IM) and other media, and facilitates presence and location-based services. SIP's extensibility and versatility enable rapid innovation of new, rich features, and rapid deployment, and it has become a market enabler for VoIP PBX's and IP telephony devices and applications. However, SIP's extensibility has also introduced interoperability challenges as vendors differentiate by extending beyond baseline SIP specifications. Implementation of rich features in a standard, interoperable manner requires ongoing standardization and industry consensus.