While trying to register to Cisco UCM, a Cisco IP phone reports TFTP authorization failure. Which cause of this issue is true?

A cabling issue would cause a physical layer failure or loss of connectivity, not an authorization-level error specific to TFTP.

A firewall blocking access would result in a connection timeout or unreachable error, not an authorization failure message.

The Certificate Trust List (CTL) file contains the list of trusted TFTP servers that a Cisco IP phone is authorized to contact. If the TFTP server's address is missing from the CTL file, the phone will reject the connection and report a TFTP authorization failure. This is a security mechanism in Cisco Unified CM to prevent phones from downloading configuration files from untrusted sources.

An incorrect DHCP option 150 address would prevent the phone from locating the TFTP server at all, producing a discovery failure rather than an authorization failure after contact is made.