What is the cause of this issue and how should it be resolved?

Refer to the exhibit.

An administrator is attempting to register a SIP phone to a Cisco UCM but the registration is failing. The IP address of the SIP Phone is 10.117.34.222 and the IP address of the Cisco UCM is 10.0.101.10. Pings from the SIP phone to the Cisco UCM are successful.

What is the cause of this issue and how should it be resolved?
A . An NTP mismatch is preventing the connection of the TCP session between the SIP phone and the Cisco UCM. The SIP phone and Cisco UCM must be set with identical NTP sources.
B. The certificates on the SIP phone are not trusted by the Cisco UCM. The SIP phone must generate new certificates.
C. DNS lookup for the Cisco UCM FQDN is failing. The SIP phone must be reconfigured with the proper DNS server.
D. An network device is blocking TCP port 5060 from the SIP phone to the Cisco UCM. This device must be reconfigured to allow traffic from the IP phone.

View Answer

Answer: D