IME, the DOT may forward your complaint to the carrier and asked to be copied on carrier's response to you. Carrier has 30 days to acknowledge and 60 days 'to provide a substantive response.' You may not think the response is adequate. The DOT doesn't (generally) pursue individual claims but instead looks at enforcement action after seeing a pattern of complaints. That's the reason why one should take the time to make a thoughtful complaint. Rambling and venting are valueless here.