Follow these instructions to factory reset your Cisco phone:
  • Remove the ethernet (network) cable.
  • Wait one minute.
  • Replace the ethernet cable.
  • Press and hold the # key within ten seconds.
  • The Message Waiting Light will start to flash. Release the # key.
  • Press the following keys in sequence: 123456789*0#
  • Wait for about twenty minutes for the reset to complete.

Watch the video below for visual instructions.