Airline refused a credit after cancelling international flight - advice on what to do next?

Airline cancelled our non-stop international flight and put us on an indirect flight, with double the travel time. We called to say that was not acceptable to us, and they declined to provide or offer a refund (as Dept. of Transportation requires), or any discount or other accommodation.

Left with no other choice, we had to take the indirect flight. Afterwards we again contacted Customer Care who said they DID offer the required refund during our prior recorded call. I asked for the recording and they said they would not share it because of “privacy and security.” Obviously there is no “privacy or security” interest in a giving me a recording of a phone call with me.

The airline considers this matter “closed.” I asked them to preserve the recording because I’d still like to get it.

Any advice on next steps? Has anyone worked through the DOT or FAA or anyone else to enforce airline regulations? Thank you!