An IP phone that is connected through a Cisco Catalyst 3750 Series Switch is failing to register with the subscriber as a backup server. When the user presses the settings button on the phone, only the Cisco Unified Communications Manager publisher shows as registered. What is the most likely cause for this issue?
A. The phone does not have the correct Cisco Unified Communications Manager group in the device configuration page.
B. The Cisco Unified Communications Manager group that is applied through the device pool is misconfigured.
C. The ip-helper address command for the subscriber is not configured on the switch port.
D. The subscriber does not have the correct device pool configured.
E. The enterprise phone configuration does not have the call control redundancy enabled.
