Exceeding customers' expectations becomes the goal whenever service providers develop products or services based on the needs of their customers. From the perspective of service science, value co-creation or interactions between the service provider and the customer create a value proposition mutual to both service system entities. Utilizing the infrastructure of the interact-service-proposal-agreement-realized (ISPAR) model, we define a methodology for gauging value co-creation that emphasizes both interactivity and value proposition realization as critical factors. We conduct a comparative case study of the product development cycle of two service applications, strategic call management and custom call flow framework. Results indicate value co-creation is achieved when a high degree of interactivity is maintained throughout the product development cycle, resulting in a value proposition realization rate.