Telecommunication systems should have a high degree of availability; i.e.; high probability of correct provision of requested services. To achieve this; correctness of software for such systems and system fault tolerance should be ensured. In this paper we show how to formalise and extend Lyra ‚Äď a top-down service-oriented method for development of communicating systems. In particular; we focus on integration of fault tolerance mechanisms into the entire Lyra development flow.