A voice engineer is moving an IP phone from one secure Cisco Unified Communications Manager cluster to another. The phones are not registering to the new cluster. Which task resolves this issue?

Resetting a phone restores factory defaults but does not specifically target or remove the CTL file, so the cluster trust mismatch that is blocking registration persists.

Restarting the phone reloads its current configuration from flash memory but does not delete the old cluster's CTL file, leaving the root cause of the registration failure unresolved.

Each Cisco IP phone stores a CTL file containing the security certificates of the cluster it originally enrolled with. When a phone is moved to a different CUCM cluster, the existing CTL file causes the phone to reject the new cluster's certificates, blocking registration entirely. Manually deleting the CTL file forces the phone to obtain a fresh CTL from the new cluster, resolving the trust mismatch and allowing registration to complete.

Importing phones via the Bulk Administration Tool adds records to the new cluster's database, but the phones still hold the old CTL file locally and will refuse to register until that file is cleared.