In a cluster system, if a node fails, then stable calls (those in conversation state) will be preserved but unstable calls may be dropped. If the failing node is the last active node in the system, then all ongoing calls may be dropped.
On total failures, billing is most likely not lost because one of the four hard drives should still have stored billing records to be read later.
There are 3 failover scenarios related to the 1+1 redundancy:
Node Failure:When a Lotus Sametime Unified Telephony node fails, the other node takes over. The MGCP signaling manager on the partner node continues to use the same media server as it did before the node failure took place.
Media server failure: When the primary media server fails, the Lotus Sametime Unified Telephony automatically switches the media server traffic to the backup media server.
Site or building failure: When there is a total failure of site A, the partner Lotus Sametime Unified Telephony node detects this event and starts using the site B media server.
Parent topic: Survivability Overview
Media Server Survivability with 1+1 Redundancy