[Serusers] How Many Nines?

Hello, I'm looking at a project which requires a higher than normal reliability for a SIP server. The project involves risk-of-life communications. The volume of traffic is quite low. I have been using SER in embedded applications for some time now without problems but I am now looking at a larger scale project. Assuming I can provide hardware server(s) with suitable characteristics, then : - How reliable is SER? - In each version? - In each module? Are there any well established high availability configurations for SER? Are there any risk-of-life deployments I can reference? We may well use SEMS for part of the project so the same questions apply. I'm not expecting 100% reliability, just a system that matches or exceeds present high reliability PABX deployments. If you are of the honest opinion that SER is not good enough, what SIP server products should I be looking at? Or is SIP not suitable full stop? Thanks in advance Jeremy