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

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