A Cisco IP phone fails to register with the Cisco CallManager. Which choice should be checked first to isolate the issue?

CDP assists with voice VLAN discovery but is not required for registration - a phone can register without CDP if the VLAN is statically configured.

Before a Cisco IP phone can contact a TFTP server or register with CallManager, it must first obtain an IP address via DHCP. If the phone is not generating DHCP request packets, or if those requests are not being answered, all subsequent steps will fail. Checking DHCP first is the correct bottom-up troubleshooting approach for IP phone registration failures.

TFTP requests occur after DHCP is successful, so checking TFTP before confirming DHCP operation skips a foundational step.

Deleting and re-creating the phone is a configuration-level action that does not help isolate a network-layer failure and should only be considered after lower-layer causes are ruled out.