A redundant Session Initiation Protocol (SIP) call center system has two data centers each having a first and a second SIP server cooperating as a first SIP-server high availability (HA) pair, a set of SW applications executable at each data center, and a plurality of agent stations each comprising a SIP telephone and a computerized appliance executing a desktop application at each agent station. The HA pairs operate as SIP server peers, the agent SIP phones are configured to maintain simultaneous registration with both SIP server peers, and Agent Desktop applications log in to only one SIP server peer, wherein the SIP Server peers collaborate to deliver calls to individual agent SIP phones via the SIP server peer where the agent is logged in, and wherein, upon failure of either data center agent's desktop applications log in to the other data center, allowing the associated agent to continue working.