During device failover to the secondary Cisco Unified Communications Manager server, how does the phone recognize that t

[answerhappygod]

During device failover to the secondary Cisco Unified Communications Manager server, how does the phone recognize that the primary server is back?

A. The secondary server keeps sending keepalive message to the primary server and when it succeeds, it unregisters the phones to force them to register to the primary.
B. When the primary server goes online, it sends out an "ALIVE" message via broadcast so that the phones re-register.
C. The phones never re-register with the primary server until the active (secondary) one goes down.
D. The phone sends keepalive messages to the primary server frequently and when it succeeds, the phone re-registers with it.