Create High-Availability software-base SIP switches

We want to build 2 servers that support fail-over and high availability. The 2 servers operate as VoIP/SIP switches and have high-availability/fail-over capabilities. 3rth server as file/backup/database server is an option. Please put VoIP-switch in your anwser. The servers should be able to pickup from and send to endpoint linked to VoIP accounts/trunks. We need to register the exact lenght of every call and also call log for failed calls for example. We would like to know how you would approach this project, which capabilities it would have, which timeframe for completion, how to add/remove SIP-trunks + SIP-accounts would go and how the exact process work in case a server goes offline.