An IP phone that is using PoE shows "unknown" in Cisco Unified Communications Manager. What should the engineer do to restore the phone to service?

Restarting the phone from CUCM sends a signal over the network; if the underlying connectivity is broken, the restart command cannot reach the device.

Resetting the phone from CUCM performs a factory reset but still depends on network reachability, which is already impaired.

Entering **# locally initiates a factory reset sequence on the phone itself and would erase configuration without resolving a physical connectivity or PoE issue.

When CUCM displays 'Unknown' for a PoE phone, it typically means the device is not communicating properly over the network, pointing to a physical or link-layer issue. Inspecting the network cable for damage and verifying that the switch port is active, correctly configured, and delivering PoE power addresses the root cause and restores the phone to a registered state.