Evolutionary, a Next Generation Network (NGN) is a significant improvement of a successfully existing network. The third Generation Partnership Project (3GPP) aimed to merge two of the most successful paradigms in communications: cellular networks and the Internet. Within the IP Multimedia Subsystem (IMS) the 3GPP specified a comprehensive and service oriented architecture that includes session based Quality of Service in IP, charging mechanisms and offers standardized interfaces for application service integration and seamless interactions with legacy services. Under this technical premise a variety of promising value added services suppose to attend our entire communication life. Fraunhofer Institute FOKUS designed and implemented a 3GPP compliant Application Server for converged services called the "Open SIP AS" which is deploying services within the "National Host for 3Gb Applications Testbed" on top of an IMS architecture. This paper describes details of the implementation and the usage of the "Open SIP AS" and gives an example of a NGN service.