Context enabled mobile applications are considered to provide a richer experience and to enhance the user interaction by acquiring information that allows the identification of the user’s current situation. Modern context inference infrastructures can source, process and deliver user information. However, a commercialization towards a context service has still been prohibited by the need for global service coverage and accurate context identification. With the advent of Next Generation Networks, telecom operators can leverage All-IP networks to design external service interfaces that integrate a diverse set of sources and context inference processes that are easily scalable, extendable, and robust at the same time. This article presents a telecom operator service that supplies mobile applications with context information to illustrate how context infrastructures can leverage NGN capabilities. The article introduces an innovative context inference approach involving third Party applications within the inference process itself. This is done by structuring the ontology context model in layers of complexity and inferring particular context information via modules, which are designed in collaboration with third party developers. Furthermore, the service is compliant with state-of-the-art IP Multimedia Subsystem infrastructures and provides an interface that uses HTTP/SOAP and HTTP/REST communication as well as the Session Initiation Protocol of the IMS. A first proof of concept indicates an increased service adoption, a higher accuracy of context, and an increased robustness towards errors.