Question

A newly deployed batch of Cisco IP phones fails to register with a CUCM cluster running in mixed mode (secured). Phones that were already registered before enabling mixed mode continue to work normally. Packet captures from a failing phone show the TFTP download of the ITL file completing successfully, but the TLS handshake to CUCM fails with a certificate validation error. The phone's displayed date is January 1, 1970. What is the MOST likely root cause?

Accepted Answer

The phone cannot reach an NTP source, so its clock is invalid and it rejects the CUCM certificate as not yet valid

Answer

The CUCM CallManager certificate has expired and must be regenerated on all nodes

Answer

The phones are running a firmware load that does not support the ITL file format

Answer

The CTL file was never distributed to the phones, preventing any secure registration