Adapting services and applications to context bears the promise for increased user experience. Hence, context aware computing has been recognized as crucial for future, in particular mobile, services. The main focus of our work is on context aware systems, where context sources are distributed and span various domains. While a lot of work has been done already, many solutions fall short when it comes to the actual deployment, either due to complexity reasons or lack of widespread acceptance of the used technologies. The solution we developed aims for a platform that is highly extensible, achieves interoperability across devices and networks but is also grounded in available standards, making its widespread deployment possible almost immediately. The solution is rooted in two major design principles as well as in the usage of the event framework for the Session Initiation Protocol (SIP) to ensure this aim. In order to ground our platform to a specific use case, we also developed a proof-of-concept prototype within a mobile environment.