We propose an object-oriented specification language, FUS++, for expressing the functional behaviors and the desired properties of each telecommunications service at the analysis phase. Our approach is based on Fusion, an object-oriented method that consists of analysis, design, and implementation phases for software development. Accompanying FUS++ is a tool we developed for translating FUS++ specifications to Promela statements; validation of service specifications and detection of feature interactions are thus possible by applying Spin tools to these Promela statements. To ensure a correct implementation of services with respect to their specifications, we exercise a novel concept of adding a service modeler and observer (SMO) to the target system where the execution of these services takes place. Combined with a test scenario generator, SMO is quite effective in identifying implementation errors on the fly. This paper presents the FUS++ language, the specification of telecommunications services based on FUS++, the concept of SMO, and the realization of SMO in a CS-1 (Capability Set No 1) intelligent network. Index terms : telecommunication software engineering, validation, software lifecycle, property verification, intelligent network.