In recent Cisco Unified CM (CUCM) deployments, phones usually request SEP<MAC>.cnf.xml directly. XMLDefault.cnf.xml is more common in third-party TFTP servers (e.g., in small businesses using CME or Asterisk with TFTP).

is the fallback configuration file. When a Cisco IP phone boots up, it follows a strict hierarchy:

The phone can ping the TFTP server (CUCM Publisher or TFTP node), but the service isn't listening on port 69.