A collaboration engineer is troubleshooting a Cisco IP Phone 8841 that fails to register with a Cisco Unified Communications Manager (CUCM) cluster. The phone successfully obtains an IP address from DHCP, including option 150. A packet capture on the phone's SPAN port reveals that the phone sends a TFTP request for its configuration file but receives no response. The CUCM publisher and subscribers are reachable via ICMP from the same VLAN. Which service on the CUCM servers is most likely not running?