Which failure would prevent phones from registering to CUCM?

• A. DHCP server down
• B. TFTP unavailable
• C. SIP trunk down
• D. DNS resolution failure

Answer: (B)

The key idea is that phones must download their provisioning and configuration data before they can register with CUCM. When a Cisco IP phone boots, it gets an IP via DHCP and learns where to fetch its config from a TFTP server. It then downloads a device-specific config file (for example, SEP MAC.cnf.xml) from TFTP, which tells the phone how to contact CUCM and which signaling method to use. If the TFTP server is unavailable, the phone cannot retrieve this configuration, so there is nothing for the phone to register with CUCM, and registration fails.

If DHCP were down, the phone wouldn’t even obtain an IP address to reach CUCM in the first place, so registration would be blocked for that reason. A SIP trunk outage affects call placement, not the initial registration of the phones. DNS resolution failure could prevent locating servers if names are used, but many deployments use direct IP addresses for TFTP and CUCM; without the ability to fetch the config from TFTP, registration cannot proceed regardless.