When configuring a device, you have the option to set a "Failover to" device, which calls sent to the device will be sent to as a backup in the event that they can't be routed to the primary device.
This is most useful when, for example, you have redundant PBX's or call termination equipment that can both handle the same set of numbers - such as those in a High Availability team or cluster.
If the device you are configuring does not accept the call, the call will then be sent to the secondary device.
Note that you cannot chain together multiple devices - only a single failover attempt will be made per call. For example:
- Device A is offline and configured to failover to device B
- Device B is offline, but configured to failover to device C
- The call will attempt device A, then failover to B, but will not attempt device C
This is a restriction to ensure timely call handling, as well as a protection mechanism to prevent looping. A call may also be attempted via multiple CR's. For example, if CR0 cannot reach Device A or B, then CR1 may try also (if the device is configured as a SIP trunk, with the IP's of multiple CR's allowed in the remote configuration).