Can an IP phone be configured so that calls to that IP phone can be rerouted if the IP phone become unregistered?

Call Forward All redirects every inbound call regardless of the phone's registration state, meaning calls are always forwarded even when the phone is functional and registered.

CUCM does not simply play reorder tone to callers when a phone is unregistered - the CFUR feature exists specifically to prevent that outcome.

Call Forward Busy Internal handles calls when the line is actively in use and busy, not when the device has lost its TFTP or SIP/SCCP registration.

Call Forward Unregistered (CFUR) is a dedicated feature in Cisco UCM that activates specifically when a phone loses its registration, allowing calls to be redirected to a specified destination such as voicemail or another DN. This is distinct from other call forward types because it only triggers on the unregistered condition, not on busy or all-call scenarios. It is configured per line in the directory number settings within CUCM Administration.

Calls are not dropped at the gateway when a phone is unregistered - CUCM routing logic and call forward features handle the disposition of the call within the cluster.