The provision of Government services has been challenging due to their complexity. Citizens use digital channels to obtain generalized information and perform simple services, but many still prefer traditional channels for more complex requests. This paper presents the chatbot prototype SIGMA as a new digital channel for services provided in the form of dialogs. It is instantiated in the context of the Portuguese National Portal for Government services. It uses Natural Language Processing (NLP) to interpret the intentions of citizens, allowing greater expressiveness in the more complex requests. To provide services in the informative phase, the chatbot provides information structured according to the Core Public Services Vocabulary (CPSV) model. This simplifies the data model of services while making it reusable and extensible.