How does the customer ensure reachability to ITSP, so that if one device on ITSP fails, the calls will be routed to another device?

A customer is using a SIP trunk to route calls to ITSP to decrease the possibility of downtime, the customer invested in a failover device.

How does the customer ensure reachability to ITSP, so that if one device on ITSP fails, the calls will be routed to another device?
A . Enable transmit security status on the SIP security profile
B . Enable ANAT on the SIP profile.
C . Monitor the link using network management toots, and if it fails, manually change the routing to another working device.
D . Enable SIP Option Ping on the SIP profile.

Answer: D