Designing for continuous service is a challenge for every telecom application developer. There are various telecom platforms that provide frameworks to address this issue. However, these have proprietary interfaces and are often complex to develop new application. In this paper, we describe our experience with using open source cluster middleware, OpenAIS, for building a telecom application used in IP Multimedia subsystem. The telecom application is a Session Initiation Protocol (SIP) Registrar that keeps user location information In particular, we discuss important design and implementation aspects in making SIP registrar highly available. We also discuss important high availability state transitions, fault handling, application state synchronization. In the end, we share important lessons learned during design, implementation, and deployment.