What causes an IP phone to register with the SRST router when the IP WAN fails?

A.
The IP WAN interface on the SRST router goes down and triggers an SRST operation.

B.
The IP phone loses keepalives from the Cisco Unified CallManager and reregisters with the SRST.

C.
The SRST sends an INVITE message to the IP phone when the IP WAN fails.

D.
The SRST router stops receiving keepalives from the Cisco Unified CallManager and falls back to the SRST.

E.
The IP phone receives a new configuration to register with the SRST router when it becomes active.

Explanation:

When the WAN link fails, Cisco IP phones detect that they are no longer receiving keepalive packets from the Cisco CallManager. Cisco IP phones then register with the router, using Skinny Client Control Protocol over TCP port 2000. In this instance, the SRST software automatically activates and builds a local database of all Cisco IP phones attached to it, up to the maximum amount set.