While the current highly fragmented Internet of Things (IoT) ecosystem is characterized by an increasing number of platforms, their interoperability and collaboration is quite challenging to achieve, even more so due to numerous standardization initiatives. However, interoperability remains essential for future IoT deployments to facilitate the emergence of novel cross-domain applications and business opportunities. In this paper we present the interoperability approach pursued by the H2020 project symbIoTe which aims at creating a flexible interoperability framework supporting both semantic and organizational interoperability. While semantic interoperability, as a prerequisite for platform cooperation, has been widely addressed in literature, symbIoTe goes a step further to propose novel aspects of organizational interoperability by introducing a concept of IoT platform federations and roaming IoT devices. We present the symbIoTe architecture, highlight its major technical contributions, and provide an overview of current system implementation with focus on Cloud-based services which create a new perspective on IoT interoperability.