Which event triggers a Survivable Communication Manager (CM) to accept IP phones registration requests, and...

A Survivable Communication Manager (CM) is a Communication Manager instance that provides local call processing and trunking capabilities for remote sites or branches in case of a WAN outage or loss of connectivity with the core servers. A Survivable CM monitors thestatus of the main CM by sending and receiving heartbeat messages at regular intervals. A heartbeat message is a SIP OPTIONS request that tests the availability of the main CM. If the Survivable CM does not receive a response to its heartbeat message within a specified timeout period, it assumes that the main CM is down and triggers a failover process. During the failover process, the Survivable CM accepts IP phone registration requests from the local endpoints and puts the configured IP trunks in a working condition, allowing the remote site or branch to continue making and receiving calls